ENH: get_rename_function: add support for __getitem__ override

[philippefutureboy]

Is your feature request related to a problem?

I would like to be able to use a subclass of UserDict with custom __getitem__ implementation as a mapper for my dataframe columns. My use case is that I want to have a mapper with default remapping rules when the key is not present.
Unfortunately, as the implementation checks for membership of the key before actually calling __getitem__, I am only able to do the behaviour by wrapping the UserDict with a lambda.

Current implementation of get_rename_function in pandas:

def get_rename_function(mapper):
    """
    Returns a function that will map names/labels, dependent if mapper
    is a dict, Series or just a function.
    """
    if isinstance(mapper, (abc.Mapping, ABCSeries)):
        def f(x):
            if x in mapper:
                return mapper[x]
            else:
                return x

    else:
        f = mapper

    return f

My proposition is the following:

def get_rename_function(mapper):
    """
    Returns a function that will map names/labels, dependent if mapper
    is a dict, Series or just a function.
    """
    if isinstance(mapper, (abc.Mapping, ABCSeries)):
        def f(k):
            return mapper.get(k, k)
    else:
        f = mapper

    return f

which functionally should result in the same behaviour.
The advantage for me is that __getitem__ is called and I can process k to return my default mapping.
I don't know however if this would have a significant performance impact. If so, alternatively I'd propose the following:

def get_rename_function(mapper):
    """
    Returns a function that will map names/labels, dependent if mapper
    is a dict, Series or just a function.
    """
    if not callable(mapper) and isinstance(mapper, (abc.Mapping, ABCSeries)):
        def f(x):
            if x in mapper:
                return mapper[x]
            else:
                return x
    else:
        f = mapper

    return f

Then I could implement the __call__ method. The downside however is that this could be a breaking change for some people that pass a subclass of a Mapping as well.

What do you think?

[jreback]

we have an errors kwarg that would do what u want

this proposal is a breaking change

[philippefutureboy]

Hey @jreback !

Thanks for your answer.

Which errors kwarg do you refer to? My use case is specifically with DataFrame.rename.

I think I understand why this is a breaking change. It's because we can't assume that calling getitem is non breaking cause we don't know what is the implementation of the client right?

[jbrockmendel]

I'm not 100% clear on the desired end-result. Is this similar to #11723?

[philippefutureboy]

Well long story short I wanted to skip the call to __contains__ and use .get directly instead.

And in terms of the need, yes! That's exactly what I want 😄🙌

[mroeschke]

Since #11723 seems like the aligned feature request, going to close this issue and encourage further discussion and support to happen in that issue