`pd.groupby` seems to mutate my `pd.Grouper` in-place

[alichaudry]

Code Sample (copy-pastable)

import pandas as pd

df1 = pd.DataFrame([["a", 1, 2, "05/29/2019"], ["a", 4, 5, "05/28/2019"], ["b", 2, 3, "05/27/2019"]], columns=["type", "num1", "num2", "date"]).assign(date=lambda df: pd.to_datetime(df["date"]))
df2 = pd.DataFrame(columns=["type", "num1", "num2", "date"]).assign(date=lambda df: pd.to_datetime(df["date"]))

groupbys = ["type", pd.Grouper(key="date", freq="1D")]

df1.groupby(groupbys).head()
df2.groupby(groupbys).head()

Problem description

The above code sample throws the following error:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-1-ae6b527f8edf> in <module>
      7 
      8 df1.groupby(groupbys).head()
----> 9 df2.groupby(groupbys).head()

~/miniconda3/envs/mbc/lib/python3.6/site-packages/pandas/core/groupby/groupby.py in head(self, n)
   2062         """
   2063         self._reset_group_selection()
-> 2064         mask = self._cumcount_array() < n
   2065         return self._selected_obj[mask]
   2066 

~/miniconda3/envs/mbc/lib/python3.6/site-packages/pandas/core/groupby/groupby.py in _cumcount_array(self, ascending)
    730         (though the default is sort=True) for groupby in general
    731         """
--> 732         ids, _, ngroups = self.grouper.group_info
    733         sorter = get_group_index_sorter(ids, ngroups)
    734         ids, count = ids[sorter], len(ids)

pandas/_libs/properties.pyx in pandas._libs.properties.CachedProperty.__get__()

~/miniconda3/envs/mbc/lib/python3.6/site-packages/pandas/core/groupby/ops.py in group_info(self)
    250     @cache_readonly
    251     def group_info(self):
--> 252         comp_ids, obs_group_ids = self._get_compressed_labels()
    253 
    254         ngroups = len(obs_group_ids)

~/miniconda3/envs/mbc/lib/python3.6/site-packages/pandas/core/groupby/ops.py in _get_compressed_labels(self)
    266 
    267     def _get_compressed_labels(self):
--> 268         all_labels = [ping.labels for ping in self.groupings]
    269         if len(all_labels) > 1:
    270             group_index = get_group_index(all_labels, self.shape,

~/miniconda3/envs/mbc/lib/python3.6/site-packages/pandas/core/groupby/ops.py in <listcomp>(.0)
    266 
    267     def _get_compressed_labels(self):
--> 268         all_labels = [ping.labels for ping in self.groupings]
    269         if len(all_labels) > 1:
    270             group_index = get_group_index(all_labels, self.shape,

~/miniconda3/envs/mbc/lib/python3.6/site-packages/pandas/core/groupby/grouper.py in labels(self)
    365     def labels(self):
    366         if self._labels is None:
--> 367             self._make_labels()
    368         return self._labels
    369 

~/miniconda3/envs/mbc/lib/python3.6/site-packages/pandas/core/groupby/grouper.py in _make_labels(self)
    386             # we have a list of groupers
    387             if isinstance(self.grouper, BaseGrouper):
--> 388                 labels = self.grouper.label_info
    389                 uniques = self.grouper.result_index
    390             else:

pandas/_libs/properties.pyx in pandas._libs.properties.CachedProperty.__get__()

~/miniconda3/envs/mbc/lib/python3.6/site-packages/pandas/core/groupby/ops.py in label_info(self)
    261         labels, _, _ = self.group_info
    262         if self.indexer is not None:
--> 263             sorter = np.lexsort((labels, self.indexer))
    264             labels = labels[sorter]
    265         return labels

ValueError: all keys need to be the same shape

The pd.Grouper object seems to be modified inside the list. This is not the behavior I expect. I can resolve this by making explicit (deep) copies of the list so that new instances of pd.Grouper are passed into both groupby methods. Like so:

In [11]: import copy                                                                                                                        

In [12]: groupbys = ["type", pd.Grouper(key="date", freq="1D")]                                                                             

In [13]: df1.groupby(copy.deepcopy(groupbys)).head()                                                                                        
Out[13]: 
  type  num1  num2       date
0    a     1     2 2019-05-29
1    a     4     5 2019-05-28
2    b     2     3 2019-05-27

In [14]: df2.groupby(copy.deepcopy(groupbys)).head()                                                                                        
Out[14]: 
Empty DataFrame
Columns: [type, num1, num2, date]
Index: []

Expected Output

As show above, expected output is the empty dataframe, instead of the ValueError. Interestingly, if I reverse the order and run the df2.groupby first, then run df1.groupby, it works fine. However, doing df2.groupby again throws the ValueError. There's definitely something in the df1.groupby that is modifying the pd.Grouper.

Output of pd.show_versions()

[paste the output of pd.show_versions() here below this line]

INSTALLED VERSIONS

commit: None
python: 3.6.7.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-50-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.2
pytest: 4.4.0
pip: 19.0.3
setuptools: 40.8.0
Cython: None
numpy: 1.16.2
scipy: 1.2.1
pyarrow: None
xarray: None
IPython: 7.4.0
sphinx: None
patsy: None
dateutil: 2.8.0
pytz: 2018.9
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 3.0.3
openpyxl: None
xlrd: 1.2.0
xlwt: None
xlsxwriter: 1.1.6
lxml.etree: None
bs4: None
html5lib: None
sqlalchemy: 1.2.18
pymysql: None
psycopg2: 2.8.1 (dt dec pq3 ext lo64)
jinja2: 2.10.1
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

If this is expected behavior please let me know (we can close the issue). If this is not expected behavior, I'd love to take a crack at resolving this (any insight into the issue would be appreciated).

[jreback]

this is probably inadvertant, so a PR to fix would be great.

note that more typically you would do

g = df.groupby(....)
g...
g...

IOW you only really would use this once

[alichaudry]

1. I just opened a PR to attempt to resolve this. Does BUG: add reset logic for Grouper if new obj is passed in (#26564) #29800 seem too naive of a fix for this bug?
2. Does this fix need a test?
3. I'm not really sure which whatsnew to add to.

[alichaudry]

I see now that the updates needed for this are a little bit more complex than I thought. To ensure nothing else breaks more thought will have to go into this.