# Buggy code
def execute(cls, ctx, op):
    a = ctx[op.inputs[0].key]

    n = op.n_partition
    w = int(a.shape[op.axis] // n)

    slc = (slice(None),) * op.axis + (slice(0, n * w, w),)
    if op.sort_type == "sort_values":
        ctx[op.outputs[0].key] = res = execute_sort_values(a, op)
        # do regular sample
        if op.by is not None:
            ctx[op.outputs[-1].key] = res[op.by].iloc[slc]
        else:
            ctx[op.outputs[-1].key] = res.iloc[slc]
    else:
        ctx[op.outputs[0].key] = res = execute_sort_index(a, op)
        # do regular sample
        ctx[op.outputs[-1].key] = res.iloc[slc]


# Traceback (AttributeError)
In [25]: merge_df = parsing_df.append(pFold_4_df)

In [26]: mDf_g = merge_df.groupby(["query","template"])

In [27]: mDf_g.execute()
Out[27]: ---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
~/anaconda3/envs/py37/lib/python3.7/site-packages/IPython/core/formatters.py in __call__(self, obj)
700                 type_pprinters=self.type_printers,
701                 deferred_pprinters=self.deferred_printers)
--> 702             printer.pretty(obj)
703             printer.flush()
704             return stream.getvalue()

~/anaconda3/envs/py37/lib/python3.7/site-packages/IPython/lib/pretty.py in pretty(self, obj)
392                         if cls is not object \
393                                 and callable(cls.__dict__.get('__repr__')):
--> 394                             return _repr_pprint(obj, self, cycle)
395
396             return _default_pprint(obj, self, cycle)

~/anaconda3/envs/py37/lib/python3.7/site-packages/IPython/lib/pretty.py in _repr_pprint(obj, p, cycle)
698     """A pprint that just redirects to the normal repr function."""
699     # Find newlines and replace them with p.break_()
--> 700     output = repr(obj)
701     lines = output.splitlines()
702     with p.group():

~/anaconda3/envs/py37/lib/python3.7/site-packages/mars/dataframe/core.py in __repr__(self)
998
999     def __repr__(self):
-> 1000         return self._to_str(representation=True)
1001
1002     def _repr_html_(self):

~/anaconda3/envs/py37/lib/python3.7/site-packages/mars/dataframe/core.py in _to_str(self, representation)
968         else:
969             corner_data = fetch_corner_data(
--> 970                 self, session=self._executed_sessions[-1])
971
972             buf = StringIO()

~/anaconda3/envs/py37/lib/python3.7/site-packages/mars/dataframe/utils.py in fetch_corner_data(df_or_series, session)
768         return df_or_series.fetch(session=session)
769     else:
--> 770         head = iloc(df_or_series)[:index_size]
771         tail = iloc(df_or_series)[-index_size:]
772         head_data, tail_data = \

~/anaconda3/envs/py37/lib/python3.7/site-packages/mars/dataframe/indexing/iloc.py in __getitem__(self, indexes)
111         else:
112             op = SeriesIlocGetItem(indexes=process_iloc_indexes(self._obj, indexes))
--> 113         return op(self._obj)
114
115     def __setitem__(self, indexes, value):

~/anaconda3/envs/py37/lib/python3.7/site-packages/mars/dataframe/indexing/iloc.py in __call__(self, series)
485             index_value = indexing_index_value(series.index_value, self.indexes[0])
486             inputs = [series] + [index for index in self._indexes if isinstance(index, (Base, Entity))]
--> 487             return self.new_series(inputs, shape=shape, dtype=series.dtype,
488                                    index_value=index_value, name=series.name)
489

AttributeError: 'DataFrameGroupByData' object has no attribute 'dtype'

# Fixed code
def execute(cls, ctx, op):
    a = ctx[op.inputs[0].key]

    if op.sort_type == "sort_values":
        ctx[op.outputs[0].key] = res = execute_sort_values(a, op)
    else:
        ctx[op.outputs[0].key] = res = execute_sort_index(a, op)

    n = op.n_partition
    if a.shape[op.axis] < n:
        num = n // a.shape[op.axis] + 1
        res = execute_sort_values(pd.concat([a] * num), op)
    w = int(res.shape[op.axis] // n)

    slc = (slice(None),) * op.axis + (slice(0, n * w, w),)
    if op.sort_type == "sort_values":
        # do regular sample
        if op.by is not None:
            ctx[op.outputs[-1].key] = res[op.by].iloc[slc]
        else:
            ctx[op.outputs[-1].key] = res.iloc[slc]
    else:
        # do regular sample
        ctx[op.outputs[-1].key] = res.iloc[slc]