Issue 21801: inspect.signature doesn't always return a signature

Hello. I noticed the following behaviour while working with xmlrpc proxy methods.

>>> import inspect, xmlrpc.client
>>> proxy = xmlrpc.client.ServerProxy('http://localhost:8000')
>>> proxy.mul
<xmlrpc.client._Method object at 0x03B0C890>
>>> inspect.signature(proxy.mul)
<xmlrpc.client._Method object at 0x03B0CC90>
>>>


Now, according to the documentation, inspect.signature should return a signature or fail, but in this case it returns the actual object, which is misleading at least. This happens because _Method implements a proxy __getattr__, any accessed attribute becoming again a _Method. At the same time, inspect.signature happily uses 

try:
   obj.__signature__
except AttributeError:
   ...

to obtain the signature, if present.

The attached patch checks if __signature__ is an actual Signature object. I searched, but couldn't find any, if __signature__ can be anything, so I hope that this approach works.

This behaviour is indeed a bug. However, I think that the solution you propose is wrong.

If we ignore invalid contents of __signature__ we are masking a bug or incorrect behaviour. In this case, you should have checked the requested attribute name in '__getattr__', and return something other than _Method, if it is a '__signature__'.

Please find attached a patch, that checks if __signature__ is an instance of Signature class, and raises a TypeError if it isn't.

Your patch looks good to me.

New changeset cc0f5d6ccb70 by Yury Selivanov in branch '3.4':
inspect: Validate that __signature__ is None or an instance of Signature.
http://hg.python.org/cpython/rev/cc0f5d6ccb70

New changeset fa5b985f0920 by Yury Selivanov in branch 'default':
inspect: Validate that __signature__ is None or an instance of Signature.
http://hg.python.org/cpython/rev/fa5b985f0920

Fixed in 3.4 and 3.5.
Thanks for the bug report!

nosy: + python-dev
messages: + msg221368

resolution: fixed
stage: resolved