False positive W0108 (unnecessary lambda)

[thehcma]

Steps to reproduce

lambda x: tuple(x)

Current behavior

This will be flagged as an unnecessary lambda, but note that, while we are converting something to a tuple, tuple while appearing to be a function, isn't. This is a type cast.

This comes up in practice, for instance, when using Pandas.

The following does not work (precisely because tuple is not a function):

grouped_df = df.groupby(group_by_attributes, as_index=False).aggregate(tuple)

The workaround:

grouped_df = df.groupby(group_by_attributes, as_index=False).aggregate(lambda x: tuple(x))

Leads to W0108 in pylint.

Expected behavior

No warning should be issued, by verifying the type of what is in the lambda. :)

pylint --version output

pylint 1.7.2,
astroid 1.5.3
Python 2.7.12 (default, Nov 19 2016, 06:48:10)
[GCC 5.4.0 20160609]

[rogalski]

tuple while appearing to be a function, isn't. This is a type cast.

Is there really a difference in Python? It'll be dispatched to __call__ eventually anyway. Or is it some weird quirk on C-level implementation of Pandas?

[thehcma]

You are indeed correct - the dispatching of the functions in core/groupby.py in the aggregate function in the pandas package is, at best, iffy.

While the parameter is called func_of_funcs, it doesn't really dispatch functions.

So we can close this - I will file a bug with the pandas maintainers.

[PCManticore]

Thanks for the issue. Unfortunately this is such an edge case, that simply disabling the message locally next to the line where this happens should do the work. Definitely needs to be addressed by pandas!

[Ivanca]

One year later and this still an issue...

def first_true_result(seq, predicate, default=None):
    """ gets first callback return that evaluates as tre """
    for i in seq:
        result = predicate(i)
        if result:
            return result
    return default

first = first_true_result(dic, lambda k: dic[k] if some_string.startswith(k) else None)

Pylint cries about "lambda may be unnecessary"