Bug report

Anyone tried:

>>> import matplotlib
>>> len(matplotlib.rcParams)
0

A great example of shooting yourself in the foot with multiple inheritance. Essentially, we do

class RcParams(MutableMapping, dict):
    def __getitem__(self, key):
        return dict.__getitem__(self, key)

    def __setitem__(self, key, val):
        return dict.__setitem__(self, key, val)

but do not reimplement __len__. You can work out the consequences yourself.

Proposed solution

Do not inherit RcParams from dict, and use a private data dict instead.