Deprecate groupby() squeeze option

[dechamps]

Code Sample

import pandas as pd
print(pd.DataFrame([{
    'A': 1,
    'B': 1,
}, {
    'A': 2,
    'B': 2,
}, {
    'A': 2,
    'B': 3,
}]).groupby('A', squeeze=True).count())

Problem description

I expected .groupby(squeeze=True) to, well, squeeze, and count() to return a Series. Instead squeeze=True doesn't seem to do anything, and count() returns a DataFrame.

A workaround is to write .groupby('A').count().squeeze(), which does work.

Expected Output

A
1    1
2    2
Name: B, dtype: int64

Actual output

   B
A
1  1
2  2

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit : None python : 3.7.6.final.0 python-bits : 64 OS : Linux OS-release : 5.4.0-4-amd64 machine : x86_64 processor : byteorder : little LC_ALL : None LANG : en_GB.UTF-8 LOCALE : en_GB.UTF-8

pandas : 1.0.1
numpy : 1.17.4
pytz : 2019.3
dateutil : 2.8.1
pip : 18.1
setuptools : 44.0.0
Cython : None
pytest : 4.6.9
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.5.0
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 2.11.1
IPython : 7.12.0
pandas_datareader: None
bs4 : None
bottleneck : None
fastparquet : None
gcsfs : None
lxml.etree : 4.5.0
matplotlib : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
pytest : 4.6.9
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
xlsxwriter : None
numba : None

[TomAugspurger]

It doesn't appear to be documented, and I'm not familiar with it. @dechamps are you interested in walking through the code to see what it's intended for?

[jreback]

we should deprecate this option - i don’t think original usecases that i added are worth it

[dechamps]

It doesn't appear to be documented

Well the groupby() squeeze parameter does have documentation, it seems.

Personally I don't care much about the parameter - it's simple enough to just call squeeze() later anyway. However it is of course confusing if the parameter is there but does nothing.

[WillAyd]

+1 to deprecate as well; seems out of place as an argument here

[MarcoGorelli]

However it is of course confusing if the parameter is there but does nothing.

Here's an example of where it does something

In [2]: from pandas import DataFrame                                                                                                                                                                   

In [3]:     df3 = DataFrame( 
   ...:         [ 
   ...:             {"val1": 1, "val2": 20, 'val3': 1}, 
   ...:             {"val1": 1, "val2": 20, 'val3': 2}, 
   ...:             {"val1": 1, "val2": 20, 'val3': 3}, 
   ...:             {"val1": 1, "val2": 20, 'val3': 4}, 
   ...:         ] 
   ...:     ) 
   ...:                                                                                                                                                                                                

In [4]: df3.set_index(['val1', 'val2']).groupby(['val1', 'val2'], squeeze=True).apply(sum)                                                                                                             
Out[4]: 
val3    10
dtype: int64

In [5]: df3.set_index(['val1', 'val2']).groupby(['val1', 'val2'], squeeze=False).apply(sum)                                                                                                            
Out[5]: 
           val3
val1 val2      
1    20      10

Anyway, @dechamps , are you interested in submitting a pull request to deprecate it? If so, see https://pandas.pydata.org/docs/development/contributing.html - else, I'd happily take it and see the groupby logic simplified :)

[dechamps]

Honestly I'd be prefer you do it - I have zero familiarity with Pandas development workflows.

[mlyons-tcc]

Here's a rant: You deprecated squeeze in 1.1.0 in violation of your own deprecation policy introduced in 1.0.0.

edit: Perhaps it was intended to throw a DeprecationWarning instead of the FutureWarning that was used. FutureWarning indicates that it has already been deprecated and user is still using it.