# Buggy code
def execute(cls, ctx, op: "RemoteFunction"):
    from ..session import Session

    session = ctx.get_current_session()
    prev_default_session = Session.default

    mapping = {
        inp: ctx[inp.key]
        for inp, prepare_inp in zip(op.inputs, op.prepare_inputs)
        if prepare_inp
    }
    for to_search in [op.function_args, op.function_kwargs]:
        tileable_placeholders = find_objects(to_search, _TileablePlaceholder)
        for ph in tileable_placeholders:
            mapping[ph] = ph.tileable

    function = op.function
    function_args = replace_inputs(op.function_args, mapping)
    function_kwargs = replace_inputs(op.function_kwargs, mapping)

    # set session created from context as default one
    session.as_default()
    try:
        if isinstance(ctx, ContextBase):
            with ctx:
                result = function(*function_args, **function_kwargs)
        else:
            result = function(*function_args, **function_kwargs)
    finally:
        # set back default session
        Session._set_default_session(prev_default_session)

    if op.n_output is None:
        ctx[op.outputs[0].key] = result
    else:
        if not isinstance(result, Iterable):
            raise TypeError(
                "Specifying n_output={}, but result is not iterable, got {}".format(
                    op.n_output, result
                )
            )
        result = list(result)
        if len(result) != op.n_output:
            raise ValueError(
                "Length of return value should be {}, got {}".format(
                    op.n_output, len(result)
                )
            )
        for out, r in zip(op.outputs, result):
            ctx[out.key] = r


# 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: "RemoteFunction"):
    from ..session import Session

    session = ctx.get_current_session()
    prev_default_session = Session.default

    mapping = {
        inp: ctx[inp.key]
        for inp, prepare_inp in zip(op.inputs, op.prepare_inputs)
        if prepare_inp
    }
    for to_search in [op.function_args, op.function_kwargs]:
        tileable_placeholders = find_objects(to_search, _TileablePlaceholder)
        for ph in tileable_placeholders:
            tileable = ph.tileable
            chunk_index_to_shape = dict()
            for chunk in tileable.chunks:
                if any(np.isnan(s) for s in chunk.shape):
                    shape = ctx.get_chunk_metas(
                        [chunk.key], filter_fields=["chunk_shape"]
                    )[0][0]
                    chunk._shape = shape
                chunk_index_to_shape[chunk.index] = chunk.shape
            if any(any(np.isnan(s) for s in ns) for ns in tileable._nsplits):
                nsplits = calc_nsplits(chunk_index_to_shape)
                tileable._nsplits = nsplits
                tileable._shape = tuple(sum(ns) for ns in nsplits)
            mapping[ph] = tileable

    function = op.function
    function_args = replace_inputs(op.function_args, mapping)
    function_kwargs = replace_inputs(op.function_kwargs, mapping)

    # set session created from context as default one
    session.as_default()
    try:
        if isinstance(ctx, ContextBase):
            with ctx:
                result = function(*function_args, **function_kwargs)
        else:
            result = function(*function_args, **function_kwargs)
    finally:
        # set back default session
        Session._set_default_session(prev_default_session)

    if op.n_output is None:
        ctx[op.outputs[0].key] = result
    else:
        if not isinstance(result, Iterable):
            raise TypeError(
                "Specifying n_output={}, but result is not iterable, got {}".format(
                    op.n_output, result
                )
            )
        result = list(result)
        if len(result) != op.n_output:
            raise ValueError(
                "Length of return value should be {}, got {}".format(
                    op.n_output, len(result)
                )
            )
        for out, r in zip(op.outputs, result):
            ctx[out.key] = r