Using apply on a grouper works only if done after another operation on grouper

[Dr-Irv]

Code Sample, a copy-pastable example if possible

In [1]: import pandas as pd

In [2]: pd.__version__
Out[2]: '0.23.0rc2'

In [3]: dates = [pd.Timestamp("2016-01-0%d 12:00:00" % i, tz='US/Pacific')
   ...:          for i in range(1, 5)]
   ...: df = pd.DataFrame({'A': ['a', 'b'] * 2, 'B': dates})
   ...: grouped = df.groupby('A')
   ...:

In [4]: df
Out[4]:
   A                         B
0  a 2016-01-01 12:00:00-08:00
1  b 2016-01-02 12:00:00-08:00
2  a 2016-01-03 12:00:00-08:00
3  b 2016-01-04 12:00:00-08:00

In [5]: grouped.apply(lambda x: x.iloc[0])[0]
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\indexes\base.py in get_loc(self, key, method, tolerance)
   3062             try:
-> 3063                 return self._engine.get_loc(key)
   3064             except KeyError:

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.IndexEngine.get_loc (pandas\_libs\index.c:5720)()
    138
--> 139     cpdef get_loc(self, object val):
    140         if is_definitely_invalid_key(val):

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.IndexEngine.get_loc (pandas\_libs\index.c:5566)()
    160         try:
--> 161             return self.mapping.get_item(val)
    162         except (TypeError, ValueError):

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas\_libs\hashtable.c:22442)()
   1491
-> 1492     cpdef get_item(self, object val):
   1493         cdef khiter_t k

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas\_libs\hashtable.c:22396)()
   1499         else:
-> 1500             raise KeyError(val)
   1501

KeyError: 0

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
<ipython-input-5-2b16555d6e05> in <module>()
----> 1 grouped.apply(lambda x: x.iloc[0])[0]

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\frame.py in __getitem__(self, key)
   2685             return self._getitem_multilevel(key)
   2686         else:
-> 2687             return self._getitem_column(key)
   2688
   2689     def _getitem_column(self, key):

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\frame.py in _getitem_column(self, key)
   2692         # get column
   2693         if self.columns.is_unique:
-> 2694             return self._get_item_cache(key)
   2695
   2696         # duplicate columns & possible reduce dimensionality

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\generic.py in _get_item_cache(self, item)
   2485         res = cache.get(item)
   2486         if res is None:
-> 2487             values = self._data.get(item)
   2488             res = self._box_item_values(item, values)
   2489             cache[item] = res

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\internals.py in get(self, item, fastpath)
   4113
   4114             if not isna(item):
-> 4115                 loc = self.items.get_loc(item)
   4116             else:
   4117                 indexer = np.arange(len(self.items))[isna(self.items)]

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\core\indexes\base.py in get_loc(self, key, method, tolerance)
   3063                 return self._engine.get_loc(key)
   3064             except KeyError:
-> 3065                 return self._engine.get_loc(self._maybe_cast_indexer(key))
   3066
   3067         indexer = self.get_indexer([key], method=method, tolerance=tolerance)

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.IndexEngine.get_loc (pandas\_libs\index.c:5720)()
    137             util.set_value_at(arr, loc, value)
    138
--> 139     cpdef get_loc(self, object val):
    140         if is_definitely_invalid_key(val):
    141             raise TypeError("'{val}' is an invalid key".format(val=val))


C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\index.pyx in pandas._libs.index.IndexEngine.get_loc (pandas\_libs\index.c:5566)()
    159
    160         try:
--> 161             return self.mapping.get_item(val)
    162         except (TypeError, ValueError):
    163             raise KeyError(val)

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas\_libs\hashtable.c:22442)()
   1490                                        sizeof(uint32_t)) # flags
   1491
-> 1492     cpdef get_item(self, object val):
   1493         cdef khiter_t k
   1494         if val != val or val is None:

C:\EclipseWorkspaces\LiClipseWorkspace\pandas-dev\pandas36\pandas\_libs\hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas\_libs\hashtable.c:22396)()
   1498             return self.table.vals[k]
   1499         else:
-> 1500             raise KeyError(val)
   1501
   1502     cpdef set_item(self, object key, Py_ssize_t val):

KeyError: 0

Problem description

Related to #20949 (and moved from there at request).

Note that if you do the following, it works:

In [6]: grouped.nth(0)['B'].iloc[0]
Out[6]: Timestamp('2016-01-01 12:00:00-0800', tz='US/Pacific')

In [7]: grouped.apply(lambda x: x.iloc[0])[0]
Out[7]: Timestamp('2016-01-01 12:00:00-0800', tz='US/Pacific')

So doing one operation (in this case nth) prior to the apply then makes the apply work.

Expected Output

In [5]: grouped.apply(lambda x: x.iloc[0])[0]
Out[5]: Timestamp('2016-01-01 12:00:00-0800', tz='US/Pacific')

Output of pd.show_versions()

INSTALLED VERSIONS

commit: b02c69a
python: 3.6.4.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.23.0rc2
pytest: 3.4.0
pip: 9.0.1
setuptools: 38.5.1
Cython: 0.25.1
numpy: 1.14.1
scipy: 1.0.0
pyarrow: 0.8.0
xarray: None
IPython: 6.2.1
sphinx: 1.7.1
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2018.3
blosc: 1.5.1
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.2.0
openpyxl: 2.5.0
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.5
pymysql: 0.8.0
psycopg2: None
jinja2: 2.10
s3fs: 0.1.3
fastparquet: None
pandas_gbq: None
pandas_datareader: None

[jreback]

so after #20959 this is easily fixed

the issue is that .nth hold onto the _group_selection state (and doesn't reset it properly)

e.g.

In [7]: g = df.groupby('A')

In [8]: g.nth(0)
Out[8]: 
                          B
A                          
a 2016-01-01 12:00:00-08:00
b 2016-01-02 12:00:00-08:00

In [9]: g.apply(lambda x: x.iloc[0])
Out[9]: 
A
a   2016-01-01 12:00:00-08:00
b   2016-01-02 12:00:00-08:00
dtype: datetime64[ns, US/Pacific]

but

In [10]: g = df.groupby('A')

In [11]: g.apply(lambda x: x.iloc[0])
Out[11]: 
   A                         B
A                             
a  a 2016-01-01 12:00:00-08:00
b  b 2016-01-02 12:00:00-08:00

[Dr-Irv]

@jreback isn't this now closed because of #20959 being done?

[jreback]

no this particular example still fails because of the ordering issue with .nth. basically the code it is using is relying on the group being set (and not unsetting it), so its stil incorrect, seee comments on #20959