TST: Stateful/Wrong Test test_agg_timezone_round_trip

[jbrockmendel]

from pandas/tests/groupby/aggregate/test_other.py test_agg_timezone_round_trip:

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')
    
ts = df['B'].iloc[0]
assert ts == grouped.nth(0)['B'].iloc[0]
assert ts == grouped.head(1)['B'].iloc[0]
assert ts == grouped.first()['B'].iloc[0]
assert ts == grouped.apply(lambda x: x.iloc[0])[0]

The last assertion here is failing in a WIP branch, so I'm troubleshooting it on master and finding that it fails if I skip the first three assertions:

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')
assert ts == grouped.apply(lambda x: x.iloc[0])[0]  # <-- KeyError: 0

Current thought is that the assertion is wrong and the test along with it. Thoughts @jreback @jorisvandenbossche ? (I don't do much in this part of the code; anyone else I should be pinging for it?)

[jbrockmendel]

Another one from tests.indexing.test_datetime test_indexing_with_datetime_tz

df = DataFrame({'a': date_range('2014-01-01', periods=10, tz='UTC')})
result = df.iloc[5]
expected = Timestamp('2014-01-06 00:00:00+0000', tz='UTC', freq='D')
assert result == expected

This works in master, but I don't think it should. df.iloc[5] should be a single-entry Series like it would be for any other dtype.

[jreback]

no there is some state modification in groupby atm (there is an issue)
one of the _set_selection things iirc

[jbrockmendel]

Are you saying these tests are correct? Because if so I'm extra confused

[jorisvandenbossche]

I should look more in detail to whether the tested output is correct or not (what are the rules about the key column when doing apply?). But for the actual issue, as you notice, you can get different answers from the same call, depending on what was called before (the state modification Jeff mentions).

Running the example:

In [12]: 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 [13]: grouped.apply(lambda x: x.iloc[0])
Out[13]: 
   A                         B
A                             
a  a 2016-01-01 12:00:00-08:00
b  b 2016-01-02 12:00:00-08:00

In [14]: grouped.first()['B'] 
Out[14]: 
A
a   2016-01-01 12:00:00-08:00
b   2016-01-02 12:00:00-08:00
Name: B, dtype: datetime64[ns, US/Pacific]

In [15]: grouped.apply(lambda x: x.iloc[0]) 
Out[15]: 
A
a   2016-01-01 12:00:00-08:00
b   2016-01-02 12:00:00-08:00
dtype: datetime64[ns, US/Pacific]

which is of course obviously a bug.

But I am not fully sure whether it's the first or the second output of that call that is the desired behaviour (I think the first, meaning that the test is "incorrect")

[jbrockmendel]

@mroeschke am i remembering right that you've been working on groupby-related stuff recently? any idea how this could become stateful?

(the test mentioned in the OP has been fixed, but that fix did not address the potential for statefulness)

[mroeschke]

Sorry I've been in the weeds of rolling instead of groupby. I haven't really dug into the ops of groupby ever; just the high level methods at times.