Repository: modin-project/modin

Issue:
Simple groupby causes TypeError
Here is a failing test:

```
import pandas as pd
import modin.pandas as mpd

data = {
    "a": [1, 1, 2],
    "b": [11, 11, 22],
    "c": [111, 111, 222]
}

df = pd.DataFrame(data)
df = pd.concat([df])
ref = df.groupby(["a", "b", df["c"]]).size()
print(ref)

df = mpd.DataFrame(data)
df = mpd.concat([df])
exp = df.groupby(["a", "b", df["c"]]).size()
print(exp)
```

The reason of fail is in DataFrame.groupby code. When we see that 'by' is a list, but it doesn't hold column labels only, we compare its length to index length. If lengths match, we assume, that 'by' holds key data, not a list of key columns/Series. In the test above this assumption is wrong.

Also, this length check triggers execution for lazy frame, which we want to avoid. So, we need to first check if 'by' is a list of key columns/Series and check index length only when we have to assume, that 'by' holds key data.

Discussion:
modin-bot: <h1 align="center"><img width=7% alt="" src="https://upload.wikimedia.org/wikipedia/commons/thumb/8/86/Teamcity_Logo.png/600px-Teamcity_Logo.png">
    TeamCity Dask test results bot</h1>

<h3 align="center">Tests PASSed</h3>

<details><summary>Tests Logs</summary>


```
ines/dask/pandas_on_dask/frame/__init__.py                    0      0   100%
modin/engines/dask/pandas_on_dask/frame/axis_partition.py             27      5    81%
modin/engines/dask/pandas_on_dask/frame/data.py                       15      0   100%
modin/engines/dask/pandas_on_dask/frame/partition.py                  75     21    72%
modin/engines/dask/pandas_on_dask/frame/partition_manager.py          44     23    48%
modin/engines/dask/pandas_on_dask/io.py                               17      0   100%
modin/engines/dask/pandas_on_dask/series/__init__.py                   0      0   100%
modin/engines/dask/task_wrapper.py                                     9      0   100%
modin/engines/python/__init__.py                                       0      0   100%
modin/engines/python/pandas_on_python/__init__.py                      0      0   100%
modin/engines/python/pandas_on_python/frame/__init__.py                0      0   100%
modin/engines/python/pandas_on_python/frame/axis_partition.py         14     14     0%
modin/engines/python/pandas_on_python/frame/data.py                    4      4     0%
modin/engines/python/pandas_on_python/frame/partition.py              65     65     0%
modin/engines/python/pandas_on_python/frame/partition_manager.py       7      7     0%
modin/engines/python/pandas_on_python/io.py                            6      6     0%
modin/engines/python/pandas_on_python/series/__init__.py               0      0   100%
modin/engines/ray/__init__.py                                          0      0   100%
modin/engines/ray/generic/__init__.py                                  0      0   100%
modin/engines/ray/generic/frame/__init__.py                            0      0   100%
modin/engines/ray/generic/frame/partition_manager.py                  10     10     0%
modin/engines/ray/generic/io.py                                       14     14     0%
modin/engines/ray/generic/series/__init__.py                           0      0   100%
modin/engines/ray/pandas_on_ray/__init__.py                            0      0   100%
modin/engines/ray/pandas_on_ray/frame/__init__.py                      0      0   100%
modin/engines/ray/pandas_on_ray/frame/axis_partition.py               22     22     0%
modin/engines/ray/pandas_on_ray/frame/data.py                         11     11     0%
modin/engines/ray/pandas_on_ray/frame/partition.py                    86     86     0%
modin/engines/ray/pandas_on_ray/frame/partition_manager.py            43     43     0%
modin/engines/ray/pandas_on_ray/io.py                                 18     18     0%
modin/engines/ray/pandas_on_ray/series/__init__.py                     0      0   100%
modin/engines/ray/task_wrapper.py                                      7      7     0%
modin/engines/ray/utils.py                                            26     26     0%
modin/error_message.py                                                27      7    74%
modin/experimental/__init__.py                                         0      0   100%
modin/experimental/cloud/__init__.py                                   6      6     0%
modin/experimental/cloud/base.py                                      36     36     0%
modin/experimental/cloud/cluster.py                                   92     92     0%
modin/experimental/cloud/connection.py                                99     99     0%
modin/experimental/cloud/meta_magic.py                                54     54     0%
modin/experimental/cloud/rayscale.py                                 113    113     0%
modin/experimental/cloud/rpyc_proxy.py                               167    167     0%
modin/experimental/engines/__init__.py                                 0      0   100%
modin/experimental/engines/pandas_on_ray/__init__.py                   0      0   100%
modin/experimental/engines/pandas_on_ray/io_exp.py                    38     38     0%
modin/experimental/engines/pandas_on_ray/sql.py                       66     66     0%
modin/experimental/pandas/__init__.py                                  6      6     0%
modin/experimental/pandas/io_exp.py                                    7      7     0%
modin/pandas/__init__.py                                              51     22    57%
modin/pandas/base.py                                                1222    922    25%
modin/pandas/concat.py                                                61     51    16%
modin/pandas/dataframe.py                                            867    659    24%
modin/pandas/datetimes.py                                              7      3    57%
modin/pandas/general.py                                               53     35    34%
modin/pandas/groupby.py                                              295    203    31%
modin/pandas/indexing.py                                             213    213     0%
modin/pandas/io.py                                                   147     11    93%
modin/pandas/iterator.py                                              15      9    40%
modin/pandas/reshape.py                                               30     20    33%
modin/pandas/series.py                                               946    619    35%
modin/pandas/utils.py                                                 42      5    88%
--------------------------------------------------------------------------------------
TOTAL                                                               8559   5427    37%


= 85 passed, 7 skipped, 1 xfailed, 1 xpassed, 114 warnings in 67.27s (0:01:07) =
Closing remaining open files:test_write_pandas.hdf...donetest_write_modin.hdf...done
============================= test session starts ==============================
platform linux -- Python 3.7.7, pytest-6.0.0, py-1.9.0, pluggy-0.13.1
rootdir: /modin, configfile: setup.cfg
plugins: openfiles-0.5.0, remotedata-0.3.2, cov-2.10.0, custom-exit-code-0.3.0, forked-1.3.0, testmon-1.0.2, xdist-1.34.0
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.......................................................................!!!!!!!!!!!!!!!!!!!! <ExceptionInfo ImportError("C extension: nat_strings not built. If you want to import pandas from the source directory, you may need to run 'python setup.py build_ext --inplace --force' to build the C extensions first.") tblen=14>
!!!!!!!!!!!!!!!!!!!! <ExceptionInfo AttributeError("type object 'pandas._libs.lib.Seen' has no attribute '__reduce_cython__'") tblen=18>
INTERNALERROR> Traceback (most recent call last):
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pandas/__init__.py", line 32, in <module>
INTERNALERROR>     from pandas._libs import hashtable as _hashtable, lib as _lib, tslib as _tslib
INTERNALERROR>   File "pandas/_libs/lib.pyx", line 1, in init pandas._libs.lib
INTERNALERROR>   File "pandas/_libs/tslibs/timedeltas.pyx", line 1, in init pandas._libs.tslibs.timedeltas
INTERNALERROR>   File "pandas/_libs/tslibs/offsets.pyx", line 1, in init pandas._libs.tslibs.offsets
INTERNALERROR>   File "pandas/_libs/tslibs/ccalendar.pyx", line 13, in init pandas._libs.tslibs.ccalendar
INTERNALERROR>   File "pandas/_libs/tslibs/strptime.pyx", line 21, in init pandas._libs.tslibs.strptime
INTERNALERROR> ImportError: cannot import name nat_strings
INTERNALERROR> 
INTERNALERROR> During handling of the above exception, another exception occurred:
INTERNALERROR> 
INTERNALERROR> Traceback (most recent call last):
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/xdist/workermanage.py", line 372, in process_from_remote
INTERNALERROR>     kwargs["warning_message_data"]
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/xdist/workermanage.py", line 405, in unserialize_warning_message
INTERNALERROR>     mod = importlib.import_module(data["message_module"])
INTERNALERROR>   File "/opt/conda/lib/python3.7/importlib/__init__.py", line 127, in import_module
INTERNALERROR>     return _bootstrap._gcd_import(name[level:], package, level)
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 983, in _find_and_load
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 953, in _find_and_load_unlocked
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 983, in _find_and_load
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
INTERNALERROR>   File "<frozen importlib._bootstrap_external>", line 728, in exec_module
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pandas/__init__.py", line 37, in <module>
INTERNALERROR>     f"C extension: {module} not built. If you want to import "
INTERNALERROR> ImportError: C extension: nat_strings not built. If you want to import pandas from the source directory, you may need to run 'python setup.py build_ext --inplace --force' to build the C extensions first.
INTERNALERROR> Traceback (most recent call last):
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/xdist/workermanage.py", line 372, in process_from_remote
INTERNALERROR>     kwargs["warning_message_data"]
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/xdist/workermanage.py", line 405, in unserialize_warning_message
INTERNALERROR>     mod = importlib.import_module(data["message_module"])
INTERNALERROR>   File "/opt/conda/lib/python3.7/importlib/__init__.py", line 127, in import_module
INTERNALERROR>     return _bootstrap._gcd_import(name[level:], package, level)
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 983, in _find_and_load
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
INTERNALERROR>   File "<frozen importlib._bootstrap_external>", line 728, in exec_module
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pandas/core/common.py", line 16, in <module>
INTERNALERROR>     from pandas._libs import lib, tslibs
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pandas/_libs/__init__.py", line 3, in <module>
INTERNALERROR>     from .tslibs import (
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pandas/_libs/tslibs/__init__.py", line 3, in <module>
INTERNALERROR>     from .conversion import localize_pydatetime, normalize_date
INTERNALERROR>   File "pandas/_libs/tslibs/c_timestamp.pxd", line 7, in init pandas._libs.tslibs.conversion
INTERNALERROR>   File "pandas/_libs/tslibs/nattype.pxd", line 11, in init pandas._libs.tslibs.c_timestamp
INTERNALERROR>   File "pandas/_libs/missing.pxd", line 13, in init pandas._libs.tslibs.nattype
INTERNALERROR>   File "pandas/_libs/missing.pyx", line 1, in init pandas._libs.missing
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pandas/__init__.py", line 32, in <module>
INTERNALERROR>     from pandas._libs import hashtable as _hashtable, lib as _lib, tslib as _tslib
INTERNALERROR>   File "pandas/_libs/lib.pyx", line 996, in init pandas._libs.lib
INTERNALERROR> AttributeError: type object 'pandas._libs.lib.Seen' has no attribute '__reduce_cython__'
[gw32] node down: <ExceptionInfo ImportError("C extension: nat_strings not built. If you want to import pandas from the source directory, you may need to run 'python setup.py build_ext --inplace --force' to build the C extensions first.") tblen=14>
F
replacing crashed worker gw32
INTERNALERROR> Traceback (most recent call last):
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/_pytest/main.py", line 240, in wrap_session
INTERNALERROR>     session.exitstatus = doit(config, session) or 0
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/_pytest/main.py", line 296, in _main
INTERNALERROR>     config.hook.pytest_runtestloop(session=session)
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pluggy/hooks.py", line 286, in __call__
INTERNALERROR>     return self._hookexec(self, self.get_hookimpls(), kwargs)
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pluggy/manager.py", line 93, in _hookexec
INTERNALERROR>     return self._inner_hookexec(hook, methods, kwargs)
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pluggy/manager.py", line 87, in <lambda>
INTERNALERROR>     firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pluggy/callers.py", line 203, in _multicall
INTERNALERROR>     gen.send(outcome)
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pluggy/callers.py", line 80, in get_result
INTERNALERROR>     raise ex[1].with_traceback(ex[2])
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pluggy/callers.py", line 187, in _multicall
INTERNALERROR>     res = hook_impl.function(*args)
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/xdist/dsession.py", line 112, in pytest_runtestloop
INTERNALERROR>     self.loop_once()
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/xdist/dsession.py", line 135, in loop_once
INTERNALERROR>     call(**kwargs)
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/xdist/dsession.py", line 263, in worker_runtest_protocol_complete
INTERNALERROR>     self.sched.mark_test_complete(node, item_index, duration)
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/xdist/scheduler/load.py", line 151, in mark_test_complete
INTERNALERROR>     self.node2pending[node].remove(item_index)
INTERNALERROR> KeyError: <WorkerController gw32>

= 1 failed, 17386 passed, 31 skipped, 96 xfailed, 513 xpassed, 30910 warnings in 340.47s (0:05:40) =

TOTAL                                                               8559   5427    37%


= 85 passed, 7 skipped, 1 xfailed, 1 xpassed, 114 warnings in 67.27s (0:01:07) =
Closing remaining open files:test_write_pandas.hdf...donetest_write_modin.hdf...done
============================= test session starts ==============================
platform linux -- Python 3.7.7, pytest-6.0.0, py-1.9.0, pluggy-0.13.1
rootdir: /modin, configfile: setup.cfg
plugins: openfiles-0.5.0, remotedata-0.3.2, cov-2.10.0, custom-exit-code-0.3.0, forked-1.3.0, testmon-1.0.2, xdist-1.34.0
gw0 I / gw1 I / gw2 I / gw3 I / gw4 I / gw5 I / gw6 I / gw7 I / gw8 I / gw9 I / gw10 I / gw11 I / gw12 I / gw13 I / gw14 I / gw15 I / gw16 I / gw17 I / gw18 I / gw19 I / gw20 I / gw21 I / gw22 I / gw23 I / gw24 I / gw25 I / gw26 I / gw27 I / gw28 I / gw29 I / gw30 I / gw31 I / gw32 I / gw33 I / gw34 I / gw35 I / gw36 I / gw37 I / gw38 I / gw39 I / gw40 I / gw41 I / gw42 I / gw43 I / gw44 I / gw45 I / gw46 I / gw47 I
gw0 [19238] / gw1 [19238] / gw2 [19238] / gw3 [19238] / gw4 [19238] / gw5 [19238] / gw6 [19238] / gw7 [19238] / gw8 [19238] / gw9 [19238] / gw10 [19238] / gw11 [19238] / gw12 [19238] / gw13 [19238] / gw14 [19238] / gw15 [19238] / gw16 [19238] / gw17 [19238] / gw18 [19238] / gw19 [19238] / gw20 [19238] / gw21 [19238] / gw22 [19238] / gw23 [19238] / gw24 [19238] / gw25 [19238] / gw26 [19238] / gw27 [19238] / gw28 [19238] / gw29 [19238] / gw30 [19238] / gw31 [19238] / gw32 [19238] / gw33 [19238] / gw34 [19238] / gw35 [19238] / gw36 [19238] / gw37 [19238] / gw38 [19238] / gw39 [19238] / gw40 [19238] / gw41 [19238] / gw42 [19238] / gw43 [19238] / gw44 [19238] / gw45 [19238] / gw46 [19238] / gw47 [19238]

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.......................................................................!!!!!!!!!!!!!!!!!!!! <ExceptionInfo ImportError("C extension: nat_strings not built. If you want to import pandas from the source directory, you may need to run 'python setup.py build_ext --inplace --force' to build the C extensions first.") tblen=14>
!!!!!!!!!!!!!!!!!!!! <ExceptionInfo AttributeError("type object 'pandas._libs.lib.Seen' has no attribute '__reduce_cython__'") tblen=18>
INTERNALERROR> Traceback (most recent call last):
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pandas/__init__.py", line 32, in <module>
INTERNALERROR>     from pandas._libs import hashtable as _hashtable, lib as _lib, tslib as _tslib
INTERNALERROR>   File "pandas/_libs/lib.pyx", line 1, in init pandas._libs.lib
INTERNALERROR>   File "pandas/_libs/tslibs/timedeltas.pyx", line 1, in init pandas._libs.tslibs.timedeltas
INTERNALERROR>   File "pandas/_libs/tslibs/offsets.pyx", line 1, in init pandas._libs.tslibs.offsets
INTERNALERROR>   File "pandas/_libs/tslibs/ccalendar.pyx", line 13, in init pandas._libs.tslibs.ccalendar
INTERNALERROR>   File "pandas/_libs/tslibs/strptime.pyx", line 21, in init pandas._libs.tslibs.strptime
INTERNALERROR> ImportError: cannot import name nat_strings
INTERNALERROR> 
INTERNALERROR> During handling of the above exception, another exception occurred:
INTERNALERROR> 
INTERNALERROR> Traceback (most recent call last):
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/xdist/workermanage.py", line 372, in process_from_remote
INTERNALERROR>     kwargs["warning_message_data"]
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/xdist/workermanage.py", line 405, in unserialize_warning_message
INTERNALERROR>     mod = importlib.import_module(data["message_module"])
INTERNALERROR>   File "/opt/conda/lib/python3.7/importlib/__init__.py", line 127, in import_module
INTERNALERROR>     return _bootstrap._gcd_import(name[level:], package, level)
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 983, in _find_and_load
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 953, in _find_and_load_unlocked
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 983, in _find_and_load
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
INTERNALERROR>   File "<frozen importlib._bootstrap_external>", line 728, in exec_module
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pandas/__init__.py", line 37, in <module>
INTERNALERROR>     f"C extension: {module} not built. If you want to import "
INTERNALERROR> ImportError: C extension: nat_strings not built. If you want to import pandas from the source directory, you may need to run 'python setup.py build_ext --inplace --force' to build the C extensions first.
INTERNALERROR> Traceback (most recent call last):
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/xdist/workermanage.py", line 372, in process_from_remote
INTERNALERROR>     kwargs["warning_message_data"]
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/xdist/workermanage.py", line 405, in unserialize_warning_message
INTERNALERROR>     mod = importlib.import_module(data["message_module"])
INTERNALERROR>   File "/opt/conda/lib/python3.7/importlib/__init__.py", line 127, in import_module
INTERNALERROR>     return _bootstrap._gcd_import(name[level:], package, level)
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 983, in _find_and_load
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
INTERNALERROR>   File "<frozen importlib._bootstrap_external>", line 728, in exec_module
INTERNALERROR>   File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pandas/core/common.py", line 16, in <module>
INTERNALERROR>     from pandas._libs import lib, tslibs
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pandas/_libs/__init__.py", line 3, in <module>
INTERNALERROR>     from .tslibs import (
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pandas/_libs/tslibs/__init__.py", line 3, in <module>
INTERNALERROR>     from .conversion import localize_pydatetime, normalize_date
INTERNALERROR>   File "pandas/_libs/tslibs/c_timestamp.pxd", line 7, in init pandas._libs.tslibs.conversion
INTERNALERROR>   File "pandas/_libs/tslibs/nattype.pxd", line 11, in init pandas._libs.tslibs.c_timestamp
INTERNALERROR>   File "pandas/_libs/missing.pxd", line 13, in init pandas._libs.tslibs.nattype
INTERNALERROR>   File "pandas/_libs/missing.pyx", line 1, in init pandas._libs.missing
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pandas/__init__.py", line 32, in <module>
INTERNALERROR>     from pandas._libs import hashtable as _hashtable, lib as _lib, tslib as _tslib
INTERNALERROR>   File "pandas/_libs/lib.pyx", line 996, in init pandas._libs.lib
INTERNALERROR> AttributeError: type object 'pandas._libs.lib.Seen' has no attribute '__reduce_cython__'
[gw32] node down: <ExceptionInfo ImportError("C extension: nat_strings not built. If you want to import pandas from the source directory, you may need to run 'python setup.py build_ext --inplace --force' to build the C extensions first.") tblen=14>
F
replacing crashed worker gw32
INTERNALERROR> Traceback (most recent call last):
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/_pytest/main.py", line 240, in wrap_session
INTERNALERROR>     session.exitstatus = doit(config, session) or 0
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/_pytest/main.py", line 296, in _main
INTERNALERROR>     config.hook.pytest_runtestloop(session=session)
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pluggy/hooks.py", line 286, in __call__
INTERNALERROR>     return self._hookexec(self, self.get_hookimpls(), kwargs)
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pluggy/manager.py", line 93, in _hookexec
INTERNALERROR>     return self._inner_hookexec(hook, methods, kwargs)
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pluggy/manager.py", line 87, in <lambda>
INTERNALERROR>     firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pluggy/callers.py", line 203, in _multicall
INTERNALERROR>     gen.send(outcome)
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pluggy/callers.py", line 80, in get_result
INTERNALERROR>     raise ex[1].with_traceback(ex[2])
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/pluggy/callers.py", line 187, in _multicall
INTERNALERROR>     res = hook_impl.function(*args)
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/xdist/dsession.py", line 112, in pytest_runtestloop
INTERNALERROR>     self.loop_once()
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/xdist/dsession.py", line 135, in loop_once
INTERNALERROR>     call(**kwargs)
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/xdist/dsession.py", line 263, in worker_runtest_protocol_complete
INTERNALERROR>     self.sched.mark_test_complete(node, item_index, duration)
INTERNALERROR>   File "/opt/conda/lib/python3.7/site-packages/xdist/scheduler/load.py", line 151, in mark_test_complete
INTERNALERROR>     self.node2pending[node].remove(item_index)
INTERNALERROR> KeyError: <WorkerController gw32>

= 1 failed, 17386 passed, 31 skipped, 96 xfailed, 513 xpassed, 30910 warnings in 340.47s (0:05:40) =


<b>Remaining output truncated<b>


```

</details>

modin-bot: <h1 align="center"><img width=7% alt="" src="https://upload.wikimedia.org/wikipedia/commons/thumb/8/86/Teamcity_Logo.png/600px-Teamcity_Logo.png">
    TeamCity Python test results bot</h1>

<h3 align="center">Tests PASSed</h3>

<details><summary>Tests Logs</summary>


```
============================= test session starts ==============================
platform linux -- Python 3.7.7, pytest-6.0.0, py-1.9.0, pluggy-0.13.1
rootdir: /modin, configfile: setup.cfg
plugins: openfiles-0.5.0, remotedata-0.3.2, cov-2.10.0, custom-exit-code-0.3.0, forked-1.3.0, testmon-1.0.2, xdist-1.34.0
collected 94 items

modin/pandas/test/test_io.py .................sX..........s............. [ 45%]
....s..............s..s.X.....s.................ss.                      [100%]

----------- coverage: platform linux, python 3.7.7-final-0 -----------
Name                                                               Stmts   Miss  Cover
--------------------------------------------------------------------------------------
modin/__init__.py                                                     76     38    50%
modin/_version.py                                                    272    172    37%
modin/apply_license_header.py                                         19     19     0%
modin/backends/__init__.py                                             0      0   100%
modin/backends/base/__init__.py                                        0      0   100%
modin/backends/base/query_compiler.py                                130      1    99%
modin/backends/pandas/__init__.py                                      0      0   100%
modin/backends/pandas/parsers.py                                     207    175    15%
modin/backends/pandas/query_compiler.py                              845    505    40%
modin/data_management/__init__.py                                      0      0   100%
modin/data_management/dispatcher.py                                   78     16    79%
modin/data_management/factories.py                                   108     47    56%
modin/data_management/functions/__init__.py                            7      0   100%
modin/data_management/functions/binary_function.py                    21     14    33%
modin/data_management/functions/foldfunction.py                        6      1    83%
modin/data_management/functions/function.py                            6      1    83%
modin/data_management/functions/groupby_function.py                   50     44    12%
modin/data_management/functions/mapfunction.py                         6      1    83%
modin/data_management/functions/mapreducefunction.py                   9      2    78%
modin/data_management/functions/reductionfunction.py                   6      1    83%
modin/data_management/utils.py                                        32     12    62%
modin/engines/__init__.py                                              0      0   100%
modin/engines/base/__init__.py                                         0      0   100%
modin/engines/base/frame/__init__.py                                   0      0   100%
modin/engines/base/frame/axis_partition.py                            45     20    56%
modin/engines/base/frame/data.py                                     522    339    35%
modin/engines/base/frame/partition.py                                  1      0   100%
modin/engines/base/frame/partition_manager.py                        184    106    42%
modin/engines/base/io/__init__.py                                     12      0   100%
modin/engines/base/io/column_stores/__init__.py                        0      0   100%
modin/engines/base/io/column_stores/column_store_reader.py            40     29    28%
modin/engines/base/io/column_stores/feather_reader.py                  9      5    44%
modin/engines/base/io/column_stores/hdf_reader.py                      3      0   100%
modin/engines/base/io/column_stores/parquet_reader.py                 34     29    15%
modin/engines/base/io/file_reader.py                                  85     66    22%
modin/engines/base/io/io.py                                          110      6    95%
modin/engines/base/io/sql/__init__.py                                  0      0   100%
modin/engines/base/io/sql/sql_reader.py                               39     31    21%
modin/engines/base/io/text/__init__.py                                 0      0   100%
modin/engines/base/io/text/csv_reader.py                             112    105     6%
modin/engines/base/io/text/excel_reader.py                           106     97     8%
modin/engines/base/io/text/fwf_reader.py                             115    108     6%
modin/engines/base/io/text/json_reader.py                             50     43    14%
modin/engines/base/io/text/text_file_reader.py                        34     24    29%
modin/engines/base/series/__init__.py                                  0      0   100%
modin/engines/dask/__init__.py                                         0      0   100%
modin/engines/dask/pandas_on_dask/__init__.py                          0      0   100%
modin/engines/dask/pandas_on_dask/frame/__init__.py                    0      0   100%
modin/engines/dask/pandas_on_dask/frame/axis_partition.py             27     27     0%
modin/engines/dask/pandas_on_dask/frame/data.py                       15     15     0%
modin/engines/dask/pandas_on_dask/frame/partition.py                  75     75     0%
modin/engines/dask/pandas_on_dask/frame/partition_manager.py          44     44     0%
modin/engines/dask/pandas_on_dask/io.py                               17     17     0%
modin/engines/dask/pandas_on_dask/series/__init__.py                   0      0   100%
modin/engines/dask/task_wrapper.py                                     9      9     0%
modin/engines/python/__init__.py                                       0      0   100%
modin/engines/python/pandas_on_python/__init__.py                      0      0   100%
modin/engines/python/pandas_on_python/frame/__init__.py                0      0   100%
modin/engines/python/pandas_on_python/frame/axis_partition.py         14      0   100%
modin/engines/python/pandas_on_python/frame/data.py                    4      0   100%
modin/engines/python/pandas_on_python/frame/partition.py              65      6    91%
modin/engines/python/pandas_on_python/frame/partition_manager.py       7      0   100%
modin/engines/python/pandas_on_python/io.py                            6      0   100%
modin/engines/python/pandas_on_python/series/__init__.py               0      0   100%
modin/engines/ray/__init__.py                                          0      0   100%
modin/engines/ray/generic/__init__.py                                  0      0   100%
modin/engines/ray/generic/frame/__init__.py                            0      0   100%
modin/engines/ray/generic/frame/partition_manager.py                  10     10     0%
modin/engines/ray/generic/io.py                                       14     14     0%
modin/engines/ray/generic/series/__init__.py                           0      0   100%
modin/engines/ray/pandas_on_ray/__init__.py                            0      0   100%
modin/engines/ray/pandas_on_ray/frame/__init__.py                      0      0   100%
modin/engines/ray/pandas_on_ray/frame/axis_partition.py               22     22     0%
modin/engines/ray/pandas_on_ray/frame/data.py                         11     11     0%
modin/engines/ray/pandas_on_ray/frame/partition.py                    86     86     0%
modin/engines/ray/pandas_on_ray/frame/partition_manager.py            43     43     0%
modin/engines/ray/pandas_on_ray/io.py                                 18     18     0%
modin/engines/ray/pandas_on_ray/series/__init__.py                     0      0   100%
modin/engines/ray/task_wrapper.py                                      7      7     0%
modin/engines/ray/utils.py                                            26     26     0%
modin/error_message.py                                                27      8    70%
modin/experimental/__init__.py                                         0      0   100%
modin/experimental/cloud/__init__.py                                   6      6     0%
modin/experimental/cloud/base.py                                      36     36     0%
modin/experimental/cloud/cluster.py                                   92     92     0%
modin/experimental/cloud/connection.py                                99     99     0%
modin/experimental/cloud/meta_magic.py                                54     54     0%
modin/experimental/cloud/rayscale.py                                 113    113     0%
modin/experimental/cloud/rpyc_proxy.py                               167    167     0%
modin/experimental/engines/__init__.py                                 0      0   100%
modin/experimental/engines/pandas_on_ray/__init__.py                   0      0   100%
modin/experimental/engines/pandas_on_ray/io_exp.py                    38     38     0%
modin/experimental/engines/pandas_on_ray/sql.py                       66     66     0%
modin/experimental/pandas/__init__.py                                  6      6     0%
modin/experimental/pandas/io_exp.py                                    7      7     0%
modin/pandas/__init__.py                                              51     20    61%
modin/pandas/base.py                                                1222    922    25%
modin/pandas/concat.py                                                61     51    16%
modin/pandas/dataframe.py                                            867    659    24%
modin/pandas/datetimes.py                                              7      3    57%
modin/pandas/general.py                                               53     35    34%
modin/pandas/groupby.py                                              295    203    31%
modin/pandas/indexing.py                                             213    213     0%
modin/pandas/io.py                                                   147     11    93%
modin/pandas/iterator.py                                              15      9    40%
modin/pandas/reshape.py                                               30     20    33%
modin/pandas/series.py                                               946    619    35%
modin/pandas/utils.py                                                 42      5    88%
--------------------------------------------------------------------------------------
TOTAL                                                               8559   5949    30%


=========== 84 passed, 8 skipped, 2 xpassed, 146 warnings in 46.36s ============
Closing remaining open files:test_write_modin.hdf...donetest_write_pandas.hdf...done
============================= test session starts ==============================
platform linux -- Python 3.7.7, pytest-6.0.0, py-1.9.0, pluggy-0.13.1
rootdir: /modin, configfile: setup.cfg
plugins: openfiles-0.5.0, remotedata-0.3.2, cov-2.10.0, custom-exit-code-0.3.0, forked-1.3.0, testmon-1.0.2, xdist-1.34.0
gw0 I / gw1 I / gw2 I / gw3 I / gw4 I / gw5 I / gw6 I / gw7 I / gw8 I / gw9 I / gw10 I / gw11 I / gw12 I / gw13 I / gw14 I / gw15 I / gw16 I / gw17 I / gw18 I / gw19 I / gw20 I / gw21 I / gw22 I / gw23 I / gw24 I / gw25 I / gw26 I / gw27 I / gw28 I / gw29 I / gw30 I / gw31 I / gw32 I / gw33 I / gw34 I / gw35 I / gw36 I / gw37 I / gw38 I / gw39 I / gw40 I / gw41 I / gw42 I / gw43 I / gw44 I / gw45 I / gw46 I / gw47 I
gw0 [19238] / gw1 [19238] / gw2 [19238] / gw3 [19238] / gw4 [19238] / gw5 [19238] / gw6 [19238] / gw7 [19238] / gw8 [19238] / gw9 [19238] / gw10 [19238] / gw11 [19238] / gw12 [19238] / gw13 [19238] / gw14 [19238] / gw15 [19238] / gw16 [19238] / gw17 [19238] / gw18 [19238] / gw19 [19238] / gw20 [19238] / gw21 [19238] / gw22 [19238] / gw23 [19238] / gw24 [19238] / gw25 [19238] / gw26 [19238] / gw27 [19238] / gw28 [19238] / gw29 [19238] / gw30 [19238] / gw31 [19238] / gw32 [19238] / gw33 [19238] / gw34 [19238] / gw35 [19238] / gw36 [19238] / gw37 [19238] / gw38 [19238] / gw39 [19238] / gw40 [19238] / gw41 [19238] / gw42 [19238] / gw43 [19238] / gw44 [19238] / gw45 [19238] / gw46 [19238] / gw47 [19238]

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----------- coverage: platform linux, python 3.7.7-final-0 -----------
Name                                                               Stmts   Miss  Cover
--------------------------------------------------------------------------------------
modin/__init__.py                                                     76     38    50%
modin/_version.py                                                    272    172    37%
modin/apply_license_header.py                                         19     19     0%
modin/backends/__init__.py                                             0      0   100%
modin/backends/base/__init__.py                                        0      0   100%
modin/backends/base/query_compiler.py                                130      0   100%
modin/backends/pandas/__init__.py                                      0      0   100%
modin/backends/pandas/parsers.py                                     207    171    17%
modin/backends/pandas/query_compiler.py                              845     29    97%
modin/data_management/__init__.py                                      0      0   100%
modin/data_management/dispatcher.py                                   78     16    79%
modin/data_management/factories.py                                   108     47    56%
modin/data_management/functions/__init__.py                            7      0   100%
modin/data_management/functions/binary_function.py                    21      0   100%
modin/data_management/functions/foldfunction.py                        6      0   100%
modin/data_management/functions/function.py                            6      1    83%
modin/data_management/functions/groupby_function.py                   50      4    92%
modin/data_management/functions/mapfunction.py                         6      0   100%
modin/data_management/functions/mapreducefunction.py                   9      0   100%
modin/data_management/functions/reductionfunction.py                   6      0   100%
modin/data_management/utils.py                                        32      0   100%
modin/engines/__init__.py                                              0      0   100%
modin/engines/base/__init__.py                                         0      0   100%
modin/engines/base/frame/__init__.py                                   0      0   100%
modin/engines/base/frame/axis_partition.py                            45     10    78%
modin/engines/base/frame/data.py                                     522     34    93%
modin/engines/base/frame/partition.py                                  1      0   100%
modin/engines/base/frame/partition_manager.py                        184     21    89%
modin/engines/base/io/__init__.py                                     12      0   100%
modin/engines/base/io/column_stores/__init__.py                        0      0   100%
modin/engines/base/io/column_stores/column_store_reader.py            40     29    28%
modin/engines/base/io/column_stores/feather_reader.py                  9      5    44%
modin/engines/base/io/column_stores/hdf_reader.py                      3      0   100%
modin/engines/base/io/column_stores/parquet_reader.py                 34     29    15%
modin/engines/base/io/file_reader.py                                  85     66    22%
modin/engines/base/io/io.py                                          110      6    95%
modin/engines/base/io/sql/__init__.py                                  0      0   100%
modin/engines/base/io/sql/sql_reader.py                               39     31    21%
modin/engines/base/io/text/__init__.py                                 0      0   100%
modin/engines/base/io/text/csv_reader.py                             112    105     6%
modin/engines/base/io/text/excel_reader.py                           106     97     8%
modin/engines/base/io/text/fwf_reader.py                             115    108     6%
modin/engines/base/io/text/json_reader.py                             50     43    14%
modin/engines/base/io/text/text_file_reader.py                        34     24    29%
modin/engines/base/series/__init__.py                                  0      0   100%
modin/engines/dask/__init__.py                                         0      0   100%
modin/engines/dask/pandas_on_dask/__init__.py                          0      0   100%
modin/engines/dask/pandas_on_dask/frame/__init__.py                    0      0   100%
modin/engines/dask/pandas_on_dask/frame/axis_partition.py             27     27     0%
modin/engines/dask/pandas_on_dask/frame/data.py                       15     15     0%
modin/engines/dask/pandas_on_dask/frame/partition.py                  75     75     0%
modin/engines/dask/pandas_on_dask/frame/partition_manager.py          44     44     0%
modin/engines/dask/pandas_on_dask/io.py                               17     17     0%
modin/engines/dask/pandas_on_dask/series/__init__.py                   0      0   100%
modin/engines/dask/task_wrapper.py                                     9      9     0%
modin/engines/python/__init__.py                                       0      0   100%
modin/engines/python/pandas_on_python/__init__.py                      0      0   100%
modin/engines/python/pandas_on_python/frame/__init__.py                0      0   100%
modin/engines/python/pandas_on_python/frame/axis_partition.py         14      0   100%
modin/engines/python/pandas_on_python/frame/data.py                    4      0   100%
modin/engines/python/pandas_on_python/frame/partition.py              65      4    94%
modin/engines/python/pandas_on_python/frame/partition_manager.py       7      0   100%
modin/engines/python/pandas_on_python/io.py                            6      0   100%
modin/engines/python/pandas_on_python/series/__init__.py               0      0   100%
modin/engines/ray/__init__.py                                          0      0   100%
modin/engines/ray/generic/__init__.py                                  0      0   100%
modin/engines/ray/generic/frame/__init__.py                            0      0   100%
modin/engines/ray/generic/frame/partition_manager.py                  10     10     0%
modin/engines/ray/generic/io.py                                       14     14     0%
modin/engines/ray/generic/series/__init__.py                           0      0   100%
modin/engines/ray/pandas_on_ray/__init__.py                            0      0   100%
modin/engines/ray/pandas_on_ray/frame/__init__.py                      0      0   100%
modin/engines/ray/pandas_on_ray/frame/axis_partition.py               22     22     0%
modin/engines/ray/pandas_on_ray/frame/data.py                         11     11     0%
modin/engines/ray/pandas_on_ray/frame/partition.py                    86     86     0%
modin/engines/ray/pandas_on_ray/frame/partition_manager.py            43     43     0%
modin/engines/ray/pandas_on_ray/io.py                                 18     18     0%
modin/engines/ray/pandas_on_ray/series/__init__.py                     0      0   100%
modin/engines/ray/task_wrapper.py                                      7      7     0%
modin/engines/ray/utils.py                                            26     26     0%
modin/error_message.py                                                27      2    93%
modin/experimental/__init__.py                                         0      0   100%
modin/experimental/cloud/__init__.py                                   6      6     0%
modin/experimental/cloud/base.py                                      36     36     0%
modin/experimental/cloud/cluster.py                                   92     92     0%
modin/experimental/cloud/connection.py                                99     99     0%
modin/experimental/cloud/meta_magic.py                                54     54     0%
modin/experimental/cloud/rayscale.py                                 113    113     0%
modin/experimental/cloud/rpyc_proxy.py                               167    167     0%
modin/experimental/engines/__init__.py                                 0      0   100%
modin/experimental/engines/pandas_on_ray/__init__.py                   0      0   100%
modin/experimental/engines/pandas_on_ray/io_exp.py                    38     38     0%
modin/experimental/engines/pandas_on_ray/sql.py                       66     66     0%
modin/experimental/pandas/__init__.py                                  6      6     0%
modin/experimental/pandas/io_exp.py                                    7      7     0%
modin/pandas/__init__.py                                              51     20    61%
modin/pandas/base.py                                                1222     54    96%
modin/pandas/concat.py                                                61      5    92%
modin/pandas/dataframe.py                                            867     98    89%
modin/pandas/datetimes.py                                              7      0   100%
modin/pandas/general.py                                               53      3    94%
modin/pandas/groupby.py                                              295     40    86%
modin/pandas/indexing.py                                             213     33    85%
modin/pandas/io.py                                                   147     11    93%
modin/pandas/iterator.py                                              15      0   100%
modin/pandas/reshape.py                                               30      0   100%
modin/pandas/series.py                                               946     56    94%
modin/pandas/utils.py                                                 42      3    93%
--------------------------------------------------------------------------------------
TOTAL                                                               8559   2542    70%

= 18588 passed, 31 skipped, 104 xfailed, 515 xpassed, 33038 warnings in 789.56s (0:13:09) =

TOTAL                                                               8559   2542    70%

= 18588 passed, 31 skipped, 104 xfailed, 515 xpassed, 33038 warnings in 789.56s (0:13:09) =

```

</details>

modin-bot: <h1 align="center"><img width=7% alt="" src="https://upload.wikimedia.org/wikipedia/commons/thumb/8/86/Teamcity_Logo.png/600px-Teamcity_Logo.png">
    TeamCity Ray test results bot</h1>

<h3 align="center">Tests PASSed</h3>

<details><summary>Tests Logs</summary>


```
============================= test session starts ==============================
platform linux -- Python 3.7.7, pytest-6.0.0, py-1.9.0, pluggy-0.13.1
rootdir: /modin, configfile: setup.cfg
plugins: openfiles-0.5.0, remotedata-0.3.2, cov-2.10.0, custom-exit-code-0.3.0, forked-1.3.0, testmon-1.0.2, xdist-1.34.0
collected 94 items

modin/pandas/test/test_io.py .................sx..........s............. [ 45%]
...................s..s.X.....s.................ss.                      [100%]

----------- coverage: platform linux, python 3.7.7-final-0 -----------
Name                                                               Stmts   Miss  Cover
--------------------------------------------------------------------------------------
modin/__init__.py                                                     76     38    50%
modin/_version.py                                                    272    172    37%
modin/apply_license_header.py                                         19     19     0%
modin/backends/__init__.py                                             0      0   100%
modin/backends/base/__init__.py                                        0      0   100%
modin/backends/base/query_compiler.py                                130      1    99%
modin/backends/pandas/__init__.py                                      0      0   100%
modin/backends/pandas/parsers.py                                     207    119    43%
modin/backends/pandas/query_compiler.py                              845    504    40%
modin/data_management/__init__.py                                      0      0   100%
modin/data_management/dispatcher.py                                   78     16    79%
modin/data_management/factories.py                                   108     47    56%
modin/data_management/functions/__init__.py                            7      0   100%
modin/data_management/functions/binary_function.py                    21     14    33%
modin/data_management/functions/foldfunction.py                        6      1    83%
modin/data_management/functions/function.py                            6      1    83%
modin/data_management/functions/groupby_function.py                   50     44    12%
modin/data_management/functions/mapfunction.py                         6      1    83%
modin/data_management/functions/mapreducefunction.py                   9      2    78%
modin/data_management/functions/reductionfunction.py                   6      1    83%
modin/data_management/utils.py                                        32     14    56%
modin/engines/__init__.py                                              0      0   100%
modin/engines/base/__init__.py                                         0      0   100%
modin/engines/base/frame/__init__.py                                   0      0   100%
modin/engines/base/frame/axis_partition.py                            45     31    31%
modin/engines/base/frame/data.py                                     522    268    49%
modin/engines/base/frame/partition.py                                  1      0   100%
modin/engines/base/frame/partition_manager.py                        184     95    48%
modin/engines/base/io/__init__.py                                     12      0   100%
modin/engines/base/io/column_stores/__init__.py                        0      0   100%
modin/engines/base/io/column_stores/column_store_reader.py            40      0   100%
modin/engines/base/io/column_stores/feather_reader.py                  9      0   100%
modin/engines/base/io/column_stores/hdf_reader.py                      3      0   100%
modin/engines/base/io/column_stores/parquet_reader.py                 34      1    97%
modin/engines/base/io/file_reader.py                                  85      7    92%
modin/engines/base/io/io.py                                          110     45    59%
modin/engines/base/io/sql/__init__.py                                  0      0   100%
modin/engines/base/io/sql/sql_reader.py                               39      1    97%
modin/engines/base/io/text/__init__.py                                 0      0   100%
modin/engines/base/io/text/csv_reader.py                             112      4    96%
modin/engines/base/io/text/excel_reader.py                           106      9    92%
modin/engines/base/io/text/fwf_reader.py                             115     16    86%
modin/engines/base/io/text/json_reader.py                             50      2    96%
modin/engines/base/io/text/text_file_reader.py                        34      1    97%
modin/engines/base/series/__init__.py                                  0      0   100%
modin/engines/dask/__init__.py                                         0      0   100%
modin/engines/dask/pandas_on_dask/__init__.py                          0      0   100%
modin/engines/dask/pandas_on_dask/frame/__init__.py                    0      0   100%
modin/engines/dask/pandas_on_dask/frame/axis_partition.py             27     27     0%
modin/engines/dask/pandas_on_dask/frame/data.py                       15     15     0%
modin/engines/dask/pandas_on_dask/frame/partition.py                  75     75     0%
modin/engines/dask/pandas_on_dask/frame/partition_manager.py          44     44     0%
modin/engines/dask/pandas_on_dask/io.py                               17     17     0%
modin/engines/dask/pandas_on_dask/series/__init__.py                   0      0   100%
modin/engines/dask/task_wrapper.py                                     9      9     0%
modin/engines/python/__init__.py                                       0      0   100%
modin/engines/python/pandas_on_python/__init__.py                      0      0   100%
modin/engines/python/pandas_on_python/frame/__init__.py                0      0   100%
modin/engines/python/pandas_on_python/frame/axis_partition.py         14     14     0%
modin/engines/python/pandas_on_python/frame/data.py                    4      4     0%
modin/engines/python/pandas_on_python/frame/partition.py              65     65     0%
modin/engines/python/pandas_on_python/frame/partition_manager.py       7      7     0%
modin/engines/python/pandas_on_python/io.py                            6      6     0%
modin/engines/python/pandas_on_python/series/__init__.py               0      0   100%
modin/engines/ray/__init__.py                                          0      0   100%
modin/engines/ray/generic/__init__.py                                  0      0   100%
modin/engines/ray/generic/frame/__init__.py                            0      0   100%
modin/engines/ray/generic/frame/partition_manager.py                  10      5    50%
modin/engines/ray/generic/io.py                                       14      3    79%
modin/engines/ray/generic/series/__init__.py                           0      0   100%
modin/engines/ray/pandas_on_ray/__init__.py                            0      0   100%
modin/engines/ray/pandas_on_ray/frame/__init__.py                      0      0   100%
modin/engines/ray/pandas_on_ray/frame/axis_partition.py               22      1    95%
modin/engines/ray/pandas_on_ray/frame/data.py                         11      3    73%
modin/engines/ray/pandas_on_ray/frame/partition.py                    86     13    85%
modin/engines/ray/pandas_on_ray/frame/partition_manager.py            43     21    51%
modin/engines/ray/pandas_on_ray/io.py                                 18      0   100%
modin/engines/ray/pandas_on_ray/series/__init__.py                     0      0   100%
modin/engines/ray/task_wrapper.py                                      7      0   100%
modin/engines/ray/utils.py                                            26      8    69%
modin/error_message.py                                                27      7    74%
modin/experimental/__init__.py                                         0      0   100%
modin/experimental/cloud/__init__.py                                   6      6     0%
modin/experimental/cloud/base.py                                      36     36     0%
modin/experimental/cloud/cluster.py                                   92     92     0%
modin/experimental/cloud/connection.py                                99     99     0%
modin/experimental/cloud/meta_magic.py                                54     54     0%
modin/experimental/cloud/rayscale.py                                 113    113     0%
modin/experimental/cloud/rpyc_proxy.py                               167    167     0%
modin/experimental/engines/__init__.py                                 0      0   100%
modin/experimental/engines/pandas_on_ray/__init__.py                   0      0   100%
modin/experimental/engines/pandas_on_ray/io_exp.py                    38     38     0%
modin/experimental/engines/pandas_on_ray/sql.py                       66     66     0%
modin/experimental/pandas/__init__.py                                  6      6     0%
modin/experimental/pandas/io_exp.py                                    7      7     0%
modin/pandas/__init__.py                                              51     17    67%
modin/pandas/base.py                                                1222    922    25%
modin/pandas/concat.py                                                61     51    16%
modin/pandas/dataframe.py                                            867    659    24%
modin/pandas/datetimes.py                                              7      3    57%
modin/pandas/general.py                                               53     35    34%
modin/pandas/groupby.py                                              295    203    31%
modin/pandas/indexing.py                                             213    213     0%
modin/pandas/io.py                                                   147     11    93%
modin/pandas/iterator.py                                              15      9    40%
modin/pandas/reshape.py                                               30     20    33%
modin/pandas/series.py                                               946    619    35%
modin/pandas/utils.py                                                 42      5    88%
--------------------------------------------------------------------------------------
TOTAL                                                               8559   5269    38%


= 85 passed, 7 skipped, 1 xfailed, 1 xpassed, 102 warnings in 68.22s (0:01:08) =
Closing remaining open files:test_write_pandas.hdf...donetest_write_modin.hdf...done
============================= test session starts ==============================
platform linux -- Python 3.7.7, pytest-6.0.0, py-1.9.0, pluggy-0.13.1
rootdir: /modin, configfile: setup.cfg
plugins: openfiles-0.5.0, remotedata-0.3.2, cov-2.10.0, custom-exit-code-0.3.0, forked-1.3.0, testmon-1.0.2, xdist-1.34.0
gw0 I / gw1 I / gw2 I / gw3 I / gw4 I / gw5 I / gw6 I / gw7 I / gw8 I / gw9 I / gw10 I / gw11 I / gw12 I / gw13 I / gw14 I / gw15 I / gw16 I / gw17 I / gw18 I / gw19 I / gw20 I / gw21 I / gw22 I / gw23 I / gw24 I / gw25 I / gw26 I / gw27 I / gw28 I / gw29 I / gw30 I / gw31 I / gw32 I / gw33 I / gw34 I / gw35 I / gw36 I / gw37 I / gw38 I / gw39 I / gw40 I / gw41 I / gw42 I / gw43 I / gw44 I / gw45 I / gw46 I / gw47 I
gw0 [19238] / gw1 [19238] / gw2 [19238] / gw3 [19238] / gw4 [19238] / gw5 [19238] / gw6 [19238] / gw7 [19238] / gw8 [19238] / gw9 [19238] / gw10 [19238] / gw11 [19238] / gw12 [19238] / gw13 [19238] / gw14 [19238] / gw15 [19238] / gw16 [19238] / gw17 [19238] / gw18 [19238] / gw19 [19238] / gw20 [19238] / gw21 [19238] / gw22 [19238] / gw23 [19238] / gw24 [19238] / gw25 [19238] / gw26 [19238] / gw27 [19238] / gw28 [19238] / gw29 [19238] / gw30 [19238] / gw31 [19238] / gw32 [19238] / gw33 [19238] / gw34 [19238] / gw35 [19238] / gw36 [19238] / gw37 [19238] / gw38 [19238] / gw39 [19238] / gw40 [19238] / gw41 [19238] / gw42 [19238] / gw43 [19238] / gw44 [19238] / gw45 [19238] / gw46 [19238] / gw47 [19238]

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.X.....x.X.......X....XXX....XX.X.x..XX...X.xXX..XXX..X..XX.....X..XX.X. [  5%]
...x...............X......X..XX..X....X...X....X...X.X..X.XXX........... [  6%]
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..X....X.X......X..X...X.X.X....X..XX........X.X......X..X.X..X....XX... [  6%]
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..X.X.X..XX..X......X...x....X....X...............................X.x... [  9%]
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X.......X.X........X....X.........X........X.....x.X...X......X..X....... [ 10%]
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....x...X....X.....xxx..X.....XXx....X..X....xx..xX...X...XX....X..X.... [ 12%]
.XX...........X............X......X...x..X.XX..X..X.X.X..X.X....X....X.. [ 13%]
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.X.XX..X..X....X..X....X..X....XX..............X..............X....X.... [ 13%]
..XXX....X....XX..X.X.XX.X.X.X..X.....XX...X.X.XX.X........X....XX...... [ 14%]
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----------- coverage: platform linux, python 3.7.7-final-0 -----------
Name                                                               Stmts   Miss  Cover
--------------------------------------------------------------------------------------
modin/__init__.py                                                     76     38    50%
modin/_version.py                                                    272    172    37%
modin/apply_license_header.py                                         19     19     0%
modin/backends/__init__.py                                             0      0   100%
modin/backends/base/__init__.py                                        0      0   100%
modin/backends/base/query_compiler.py                                130      0   100%
modin/backends/pandas/__init__.py                                      0      0   100%
modin/backends/pandas/parsers.py                                     207    119    43%
modin/backends/pandas/query_compiler.py                              845    155    82%
modin/data_management/__init__.py                                      0      0   100%
modin/data_management/dispatcher.py                                   78     16    79%
modin/data_management/factories.py                                   108     47    56%
modin/data_management/functions/__init__.py                            7      0   100%
modin/data_management/functions/binary_function.py                    21      0   100%
modin/data_management/functions/foldfunction.py                        6      0   100%
modin/data_management/functions/function.py                            6      1    83%
modin/data_management/functions/groupby_function.py                   50     36    28%
modin/data_management/functions/mapfunction.py                         6      0   100%
modin/data_management/functions/mapreducefunction.py                   9      0   100%
modin/data_management/functions/reductionfunction.py                   6      0   100%
modin/data_management/utils.py                                        32     14    56%
modin/engines/__init__.py                                              0      0   100%
modin/engines/base/__init__.py                                         0      0   100%
modin/engines/base/frame/__init__.py                                   0      0   100%
modin/engines/base/frame/axis_partition.py                            45     30    33%
modin/engines/base/frame/data.py                                     522     42    92%
modin/engines/base/frame/partition.py                                  1      0   100%
modin/engines/base/frame/partition_manager.py                        184     38    79%
modin/engines/base/io/__init__.py                                     12      0   100%
modin/engines/base/io/column_stores/__init__.py                        0      0   100%
modin/engines/base/io/column_stores/column_store_reader.py            40      0   100%
modin/engines/base/io/column_stores/feather_reader.py                  9      0   100%
modin/engines/base/io/column_stores/hdf_reader.py                      3      0   100%
modin/engines/base/io/column_stores/parquet_reader.py                 34      1    97%
modin/engines/base/io/file_reader.py                                  85      7    92%
modin/engines/base/io/io.py                                          110     45    59%
modin/engines/base/io/sql/__init__.py                                  0      0   100%
modin/engines/base/io/sql/sql_reader.py                               39      1    97%
modin/engines/base/io/text/__init__.py                                 0      0   100%
modin/engines/base/io/text/csv_reader.py                             112      2    98%
modin/engines/base/io/text/excel_reader.py                           106      9    92%
modin/engines/base/io/text/fwf_reader.py                             115     16    86%
modin/engines/base/io/text/json_reader.py                             50      2    96%
modin/engines/base/io/text/text_file_reader.py                        34      1    97%
modin/engines/base/series/__init__.py                                  0      0   100%
modin/engines/dask/__init__.py                                         0      0   100%
modin/engines/dask/pandas_on_dask/__init__.py                          0      0   100%
modin/engines/dask/pandas_on_dask/frame/__init__.py                    0      0   100%
modin/engines/dask/pandas_on_dask/frame/axis_partition.py             27     27     0%
modin/engines/dask/pandas_on_dask/frame/data.py                       15     15     0%
modin/engines/dask/pandas_on_dask/frame/partition.py                  75     75     0%
modin/engines/dask/pandas_on_dask/frame/partition_manager.py          44     44     0%
modin/engines/dask/pandas_on_dask/io.py                               17     17     0%
modin/engines/dask/pandas_on_dask/series/__init__.py                   0      0   100%
modin/engines/dask/task_wrapper.py                                     9      9     0%
modin/engines/python/__init__.py                                       0      0   100%
modin/engines/python/pandas_on_python/__init__.py                      0      0   100%
modin/engines/python/pandas_on_python/frame/__init__.py                0      0   100%
modin/engines/python/pandas_on_python/frame/axis_partition.py         14     14     0%
modin/engines/python/pandas_on_python/frame/data.py                    4      4     0%
modin/engines/python/pandas_on_python/frame/partition.py              65     65     0%
modin/engines/python/pandas_on_python/frame/partition_manager.py       7      7     0%
modin/engines/python/pandas_on_python/io.py                            6      6     0%
modin/engines/python/pandas_on_python/series/__init__.py               0      0   100%
modin/engines/ray/__init__.py                                          0      0   100%
modin/engines/ray/generic/__init__.py                                  0      0   100%
modin/engines/ray/generic/frame/__init__.py                            0      0   100%
modin/engines/ray/generic/frame/partition_manager.py                  10      0   100%
modin/engines/ray/generic/io.py                                       14      3    79%
modin/engines/ray/generic/series/__init__.py                           0      0   100%
modin/engines/ray/pandas_on_ray/__init__.py                            0      0   100%
modin/engines/ray/pandas_on_ray/frame/__init__.py                      0      0   100%
modin/engines/ray/pandas_on_ray/frame/axis_partition.py               22      0   100%
modin/engines/ray/pandas_on_ray/frame/data.py                         11      3    73%
modin/engines/ray/pandas_on_ray/frame/partition.py                    86      7    92%
modin/engines/ray/pandas_on_ray/frame/partition_manager.py            43     16    63%
modin/engines/ray/pandas_on_ray/io.py                                 18      0   100%
modin/engines/ray/pandas_on_ray/series/__init__.py                     0      0   100%
modin/engines/ray/task_wrapper.py                                      7      0   100%
modin/engines/ray/utils.py                                            26      1    96%
modin/error_message.py                                                27      2    93%
modin/experimental/__init__.py                                         0      0   100%
modin/experimental/cloud/__init__.py                                   6      6     0%
modin/experimental/cloud/base.py                                      36     36     0%
modin/experimental/cloud/cluster.py                                   92     92     0%
modin/experimental/cloud/connection.py                                99     99     0%
modin/experimental/cloud/meta_magic.py                                54     54     0%
modin/experimental/cloud/rayscale.py                                 113    113     0%
modin/experimental/cloud/rpyc_proxy.py                               167    167     0%
modin/experimental/engines/__init__.py                                 0      0   100%
modin/experimental/engines/pandas_on_ray/__init__.py                   0      0   100%
modin/experimental/engines/pandas_on_ray/io_exp.py                    38     38     0%
modin/experimental/engines/pandas_on_ray/sql.py                       66     66     0%
modin/experimental/pandas/__init__.py                                  6      6     0%
modin/experimental/pandas/io_exp.py                                    7      7     0%
modin/pandas/__init__.py                                              51     17    67%
modin/pandas/base.py                                                1222     54    96%
modin/pandas/concat.py                                                61      5    92%
modin/pandas/dataframe.py                                            867     98    89%
modin/pandas/datetimes.py                                              7      0   100%
modin/pandas/general.py                                               53      3    94%
modin/pandas/groupby.py                                              295     40    86%
modin/pandas/indexing.py                                             213     33    85%
modin/pandas/io.py                                                   147     11    93%
modin/pandas/iterator.py                                              15      0   100%
modin/pandas/reshape.py                                               30      0   100%
modin/pandas/series.py                                               946     57    94%
modin/pandas/utils.py                                                 42      3    93%
--------------------------------------------------------------------------------------
TOTAL                                                               8559   2131    75%

= 18588 passed, 31 skipped, 104 xfailed, 515 xpassed, 33006 warnings in 945.53s (0:15:45) =

TOTAL                                                               8559   2131    75%

= 18588 passed, 31 skipped, 104 xfailed, 515 xpassed, 33006 warnings in 945.53s (0:15:45) =

```

</details>

devin-petersohn: @itamarst, this is great, exactly as I would have implemented.

A couple of notes: 
* We use `black` code formatter, running `black modin` will stop black and flake8 from complaining (you may need to `pip install black`)
* The commit messages are capped at 70 characters, I think we failed to mention that in the docs (my fault)
ienkovich: The root cause of #1770 is an incorrect assumption, that if 'by' length is equal to index length, then by holds keys for groupby (actual keys, not column names and series). This patch doesn't fix the problem, it allows the test case to go through another code branch, but the problematic code branch is still not fixed. We can modify the test case by replacing 
df.groupby(["a", "b", df["c"]]).size()
with
df.groupby(["a", "b", df["c"] + 1]).size()
and then this patch cannot help.

itamarst: That seems like a separate, additional bug then? I'll add a separate test and see if I can fix that too.
devin-petersohn: @itamarst @ienkovich I do not think the fix needs to be part of this PR because the grouping values no longer come from the data, rather they come from a transformation on the data. I have proposed in #1770 that we track that issue separately because it is not a simple fix as this is.

Side note: I also do not find it as common as this case, so we should fix the common case first to ensure we have impact for the (much more) common use case, instead of blocking on the fix for the uncommon use case. 
codecov[bot]: # [Codecov](https://codecov.io/gh/modin-project/modin/pull/1842?src=pr&el=h1) Report
> Merging [#1842](https://codecov.io/gh/modin-project/modin/pull/1842?src=pr&el=desc) into [master](https://codecov.io/gh/modin-project/modin/commit/02977df1cb1979864033aabcdd0119d78355878b&el=desc) will **decrease** coverage by `46.89%`.
> The diff coverage is `0.00%`.

[![Impacted file tree graph](https://codecov.io/gh/modin-project/modin/pull/1842/graphs/tree.svg?width=650&height=150&src=pr&token=ynEaSfgM5H)](https://codecov.io/gh/modin-project/modin/pull/1842?src=pr&el=tree)

```diff
@@             Coverage Diff             @@
##           master    #1842       +/-   ##
===========================================
- Coverage   83.58%   36.68%   -46.90%     
===========================================
  Files          79       82        +3     
  Lines        8558     8978      +420     
===========================================
- Hits         7153     3294     -3859     
- Misses       1405     5684     +4279     
```


| [Impacted Files](https://codecov.io/gh/modin-project/modin/pull/1842?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [modin/pandas/dataframe.py](https://codecov.io/gh/modin-project/modin/pull/1842/diff?src=pr&el=tree#diff-bW9kaW4vcGFuZGFzL2RhdGFmcmFtZS5weQ==) | `40.16% <0.00%> (-48.76%)` | :arrow_down: |
| [modin/engines/ray/task\_wrapper.py](https://codecov.io/gh/modin-project/modin/pull/1842/diff?src=pr&el=tree#diff-bW9kaW4vZW5naW5lcy9yYXkvdGFza193cmFwcGVyLnB5) | `0.00% <0.00%> (-100.00%)` | :arrow_down: |
| [modin/engines/dask/task\_wrapper.py](https://codecov.io/gh/modin-project/modin/pull/1842/diff?src=pr&el=tree#diff-bW9kaW4vZW5naW5lcy9kYXNrL3Rhc2tfd3JhcHBlci5weQ==) | `0.00% <0.00%> (-100.00%)` | :arrow_down: |
| [modin/experimental/pandas/io\_exp.py](https://codecov.io/gh/modin-project/modin/pull/1842/diff?src=pr&el=tree#diff-bW9kaW4vZXhwZXJpbWVudGFsL3BhbmRhcy9pb19leHAucHk=) | `0.00% <0.00%> (-100.00%)` | :arrow_down: |
| [modin/engines/ray/pandas\_on\_ray/io.py](https://codecov.io/gh/modin-project/modin/pull/1842/diff?src=pr&el=tree#diff-bW9kaW4vZW5naW5lcy9yYXkvcGFuZGFzX29uX3JheS9pby5weQ==) | `0.00% <0.00%> (-100.00%)` | :arrow_down: |
| [modin/experimental/pandas/\_\_init\_\_.py](https://codecov.io/gh/modin-project/modin/pull/1842/diff?src=pr&el=tree#diff-bW9kaW4vZXhwZXJpbWVudGFsL3BhbmRhcy9fX2luaXRfXy5weQ==) | `0.00% <0.00%> (-100.00%)` | :arrow_down: |
| [modin/engines/dask/pandas\_on\_dask/io.py](https://codecov.io/gh/modin-project/modin/pull/1842/diff?src=pr&el=tree#diff-bW9kaW4vZW5naW5lcy9kYXNrL3BhbmRhc19vbl9kYXNrL2lvLnB5) | `0.00% <0.00%> (-100.00%)` | :arrow_down: |
| [modin/engines/dask/pandas\_on\_dask/frame/data.py](https://codecov.io/gh/modin-project/modin/pull/1842/diff?src=pr&el=tree#diff-bW9kaW4vZW5naW5lcy9kYXNrL3BhbmRhc19vbl9kYXNrL2ZyYW1lL2RhdGEucHk=) | `0.00% <0.00%> (-100.00%)` | :arrow_down: |
| [...din/engines/ray/generic/frame/partition\_manager.py](https://codecov.io/gh/modin-project/modin/pull/1842/diff?src=pr&el=tree#diff-bW9kaW4vZW5naW5lcy9yYXkvZ2VuZXJpYy9mcmFtZS9wYXJ0aXRpb25fbWFuYWdlci5weQ==) | `0.00% <0.00%> (-100.00%)` | :arrow_down: |
| [.../engines/ray/pandas\_on\_ray/frame/axis\_partition.py](https://codecov.io/gh/modin-project/modin/pull/1842/diff?src=pr&el=tree#diff-bW9kaW4vZW5naW5lcy9yYXkvcGFuZGFzX29uX3JheS9mcmFtZS9heGlzX3BhcnRpdGlvbi5weQ==) | `0.00% <0.00%> (-100.00%)` | :arrow_down: |
| ... and [55 more](https://codecov.io/gh/modin-project/modin/pull/1842/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/modin-project/modin/pull/1842?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/modin-project/modin/pull/1842?src=pr&el=footer). Last update [02977df...8abd84b](https://codecov.io/gh/modin-project/modin/pull/1842?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Patch:
diff --git a/modin/pandas/dataframe.py b/modin/pandas/dataframe.py
index 68a9f05e..8cb7e2fc 100644
--- a/modin/pandas/dataframe.py
+++ b/modin/pandas/dataframe.py
@@ -440,7 +440,21 @@ class DataFrame(BasePandasDataset):
             by = by._query_compiler
         elif is_list_like(by):
             # fastpath for multi column groupby
-            if not isinstance(by, Series) and axis == 0 and all(o in self for o in by):
+            if (
+                not isinstance(by, Series)
+                and axis == 0
+                and all(
+                    (
+                        (isinstance(o, str) and (o in self))
+                        or (isinstance(o, Series) and (o._parent is self))
+                    )
+                    for o in by
+                )
+            ):
+                # We can just revert Series back to names because the parent is
+                # this dataframe:
+                by = [o.name if isinstance(o, Series) else o for o in by]
+
                 warnings.warn(
                     "Multi-column groupby is a new feature. "
                     "Please report any bugs/issues to bug_reports@modin.org."


Test patch:
diff --git a/modin/pandas/test/test_groupby.py b/modin/pandas/test/test_groupby.py
index 060906df..32c9e81a 100644
--- a/modin/pandas/test/test_groupby.py
+++ b/modin/pandas/test/test_groupby.py
@@ -1160,3 +1160,31 @@ def test_to_pandas_convertion(kwargs):
     by = ["a", "b"]
 
     eval_aggregation(*create_test_dfs(data), by=by, **kwargs)
+
+
+@pytest.mark.parametrize(
+    # When True, do df[name], otherwise just use name
+    "columns",
+    [
+        [(False, "a"), (False, "b"), (False, "c")],
+        [(False, "a"), (False, "b")],
+        [(True, "a"), (True, "b"), (True, "c")],
+        [(True, "a"), (True, "b")],
+        [(False, "a"), (False, "b"), (True, "c")],
+        [(False, "a"), (True, "c")],
+    ],
+)
+def test_mixed_columns(columns):
+    def get_columns(df):
+        return [df[name] if lookup else name for (lookup, name) in columns]
+
+    data = {"a": [1, 1, 2], "b": [11, 11, 22], "c": [111, 111, 222]}
+
+    df1 = pandas.DataFrame(data)
+    df1 = pandas.concat([df1])
+    ref = df1.groupby(get_columns(df1)).size()
+
+    df2 = pd.DataFrame(data)
+    df2 = pd.concat([df2])
+    exp = df2.groupby(get_columns(df2)).size()
+    df_equals(ref, exp)