Is IntelliJ Python 3 inspection "Expected a dictionary, got a dict" a false positive for super with **kwargs?

Question

I use Python 3 and want to wrap argparse.ArgumentParser with a custom class that sets formatter_class=argparse.RawDescriptionHelpFormatter by default. I can do this successfully, however IntelliJ IDEA 2017.1 with Python Plugin (PyCharm) gives a warning for the following code:

class CustomParser(argparse.ArgumentParser): def __init__(self, formatter_class=argparse.RawDescriptionHelpFormatter, **kwargs):     # noinspection PyArgumentList     super().__init__(formatter_class=formatter_class, **kwargs)  # warning in this line for the last argument if suppression comment above removed 

If one removes the comment with the IntelliJ suppression command the warning on kwargs is "Expected a dictionary, got a dict", however it is working. Is this a false positive warning or can this be done better without the warning? Is there a real issue behind this warning that it helps avoiding?

Side question: Is there a difference in using
formatter_class = kwargs.pop('formatter_class', argparse.RawDescriptionHelpFormatter) instead of explicitly defining the named parameter in the signature? According to PEP20 more explicit in the signature is better, right?

Answer 1

Yes, that appears to be a false positive.

You asked about formatter_class=kwargs.pop('formatter_class', argparse.RawDescriptionHelpFormatter). Please don't do that. Mutating kwargs, which appears as the next argument, seems Bad. Additionally, default keyword args should be set to a simple constant rather than some mutable datastructure, as there is a big difference between evaluating at import time and evaluating at run time. See e.g. http://www.effbot.org/zone/default-values.htm . The usual idiom would be formatter_class=None in the signature, and then in the body you test for None and you mutate kwargs to your heart's content.