My understanding was that in function calls, the keys in an **expression had to be strings. However, str.format seems to deviate from that and allows non-string keys in the mapping (and silently ignores them).

Please, see the transcript below:

>>> def f(): pass
... 
>>> def g(): pass
... 
>>> x = {None: ''}
>>> y = {1: ''}
>>> f(**x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: f() keywords must be strings
>>> f(**y)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: f() keywords must be strings
>>> g(**x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: g() keywords must be strings
>>> g(**y)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: g() keywords must be strings
>>> ''.format(**x)
''
>>> ''.format(**y)
''

I could reproduce this (incorrect?) behavior on macOS with python 3.4-3.7 and on Ubuntu 18.04 with python 3.6.