With an external grouper, there is no way to access the grouped value in a DataFrame(...).groupby(...).apply(...) workflow

groupby-apply workflows are important pandas idioms. Here’s a brief example grouping on a named DataFrame column:

>>> df = pd.DataFrame({'key': [1, 1, 1, 2, 2, 2, 3, 3, 3], 'value': range(9)})
>>> result = df.groupby('key').apply(lambda x: x['key'])
>>> result
key   
1    0    1
     1    1
     2    1
2    3    2
     4    2
     5    2
3    6    3
     7    3
     8    3
Name: key, dtype: int64

An important highlight of this example is the ability to reference the grouped value – eg, x['key'] – inside the applied function.

pandas also supports grouping on arbitrary mapping functions, iterables, and lots of other objects. In these cases, the grouped value is not represented as a named column in the DataFrame. Thus, when using apply(…), there is no apparent way to access the group key value. The only alternative is to use a (slow) for-loop solution as in:

foo = lambda _k, _g: ...
grouped = df.groupby(grouper)
result_iter = (foo(key, group) for key, group in grouped) 
key_iter = (key for key, group in grouped)
pd.DataFrame.from_records(result_iter, index=key_iter)

IMHO, the ability to access the grouped value in an idiomatic way from within the applied function is ergonomically important; the groupby-apply idiom is at best partially realized without it.

[ericabrauer]

Is there any risk of clobbering an attribute on someone’s Series?

@TomAugspurger yes 😦 Just happened to me. Not sure how I feel about an attribute with such a common name added to the dataframe which wasn’t there in the ungrouped dataframe. Was a really hard bug to pinpoint. EDIT: the problem for me was that had a column named "name" and tried accessing it with the dot access syntactic sugar. Same would happen with "key".

#25457

Could you change it to something like .name_ or .key_ for the official (non-column) attribute referencing?

[brianthelion]

It appears that .name is the attribute that I’ve been looking for! Is this the intended use-case for the attribute? If so, my feeling is that “name” is somewhat nondescript.