LookupDict does not implement full contract of the dict class

LookupDict absolutely includes all of the dict methods. In particular, it allows the use of __setitem__ and other things from the dict class. It specifically does only one thing, which is override __getitem__ to allow returning None.

So it's not ok to replace the dict with object in this case, even though the requests codebase doesn't use that functionality.

Thanks for reporting this issue though, and I hope you keep reading through the codebase!

Originally posted by @Lukasa in https://github.com/psf/requests/issues/3848#issuecomment-277197592

This is unfortunately a false statement. Take a look on the following checks:

In [1]: from requests.status_codes import codes

In [2]: codes
Out[2]: <lookup 'status_codes'>

In [3]: codes.keys()
Out[3]: dict_keys([])

In [4]: list(codes.keys())
Out[4]: []

If LookupDict pretends to quack like a duck it should do so. Or it should not derive from the dict if it works even worse:

In [8]: from requests.status_codes import codes

In [9]: codes["my_code"] = 700

In [10]: list(codes.keys())
Out[10]: ['my_code']

In [11]: codes["my_code"]

In [12]: 

Start by reproducing the requests.status_codes.codes and LookupDict examples from the issue, then trace the LookupDict implementation and its dict behavior. Clarify the intended contract before changing anything; done means the chosen contract is represented consistently by lookup, assignment, and key-enumeration behavior.