Additional calls to function when using GroupBy.apply with multiple index

[semigodking]

Code Sample, a copy-pastable example if possible

import pandas as pd
df = pd.DataFrame([[1, 2, 3], [2, 2, 3]], columns=['a', 'b', 'c'])
def t(df):
    print(df)
    print('-' * 10)
    return None

g = df.groupby(by=['a', 'b'])
g.apply(t)
print(g.groups)
g.apply(lambda x:print(x.index))

Problem description

The function is invoked by 'apply' three times while there are only two groups.

Expected Output

The function is only invoked once by 'apply' for each group.

Output of pd.show_versions()

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

commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-119-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_HK.UTF-8
LOCALE: en_HK.UTF-8

pandas: 0.21.0
pytest: 3.2.1
pip: 9.0.1
setuptools: 36.4.0
Cython: None
numpy: 1.14.3
scipy: None
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: 2.6.5
feather: None
matplotlib: None
openpyxl: 2.4.9
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 0.999999999
sqlalchemy: 1.1.15
pymysql: None
psycopg2: 2.7.3.1 (dt dec pq3 ext lo64)
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

[gfyoung]

cc @jreback

[lf-shaw]

maybe similar to #2656

[gfyoung]

@Shawlf : Thanks for pointing this out! I had a feeling this wasn't a new thing...

#2936 looks like a better one to point to.

@jreback : A duplicate perhaps?

[jorisvandenbossche]

This is a documented implementation detail that it will call the function twice on the first group. See http://pandas.pydata.org/pandas-docs/stable/generated/pandas.core.groupby.GroupBy.apply.html and red box at end of https://pandas.pydata.org/pandas-docs/stable/groupby.html#flexible-apply

(if I missed something why this case would be different, please note)

[gfyoung]

@jorisvandenbossche : Does a similar comment apply to #2936?

[jorisvandenbossche]

That is already mentioned there, and for now #2936 is kept open as the issue to potentially improve this (add a keyword to specify the path instead of trying / calling the function twice, ...)