DOC: Is sort preserved inside groupby?

[nickeubank]

Empirically, it seems to be the case that if one sorts a dataframe then takes a groupby, that sort is preserved within each groupby group. For example, it seems to be the case that my_func below always gets a df sorted by age. But I can't find any documentation on whether this is guaranteed, so I'm currently re-sorting data in my_func to be safe.

df.sort(columns = ['users', 'age'], inplace = True)
df.groupby('users').apply(my_func)

Does anyone know if this is a guaranteed behavior, and if so, might it be worth adding to docs?

This is particularly relevant for the groupby().nth() command, since if sort isn't preserved, it's not clear what it grabs!

[jreback]

dupe of #8588

sort is preserved, but this really needs some validation tests specifically for this (there are of tests that would fail if this is not the case, but we a specific tests for this).

Feel free to submit a pr / docs for this.

[nickeubank]

Thanks! Will keep an eye on this -- have a conference coming up, so can't commit just yet, but will address later if still up.