DOC: documented that .apply(func) executes func twice on the first time

[anton-d]

Related issues are #2656, #2936 and #6753.

[jreback]

good..thanks!

[jorisvandenbossche]

@anton-d An advice: try to not mix tabs and spaces for indentation.

And something else, the alignment of your code example was not fully correct so it wouldn't render well (the ..ipython:: should start at the same indentation as the text of the warning), but that are tricky rst things to get right, so no problem (I corrected it here: 95a562c). And thanks for the contribution!

[ieipi]

is this deployed ?
i still got the issue(first group called twice): with version=0.24.2
d = pd.DataFrame({"a":["x", "y"], "b":[1,2]})

def identity(df):
    print (df)
    return df

d.groupby("a").apply(identity)

outputs:

a b
0 x 1
a b
0 x 1
a b
1 y 2