inspect.signature doesn't always return a signature

[PCManticore]

BPO	21801
Nosy	@ncoghlan, @larryhastings, @PCManticore, @1st1
Files	inspect_signature.patch issue21801.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/1st1'
closed_at = <Date 2014-06-23.17:24:02.008>
created_at = <Date 2014-06-18.12:41:45.841>
labels = ['type-feature', 'library']
title = "inspect.signature doesn't always return a signature"
updated_at = <Date 2014-06-23.17:24:36.614>
user = 'https://github.com/PCManticore'

bugs.python.org fields:

activity = <Date 2014-06-23.17:24:36.614>
actor = 'yselivanov'
assignee = 'yselivanov'
closed = True
closed_date = <Date 2014-06-23.17:24:02.008>
closer = 'python-dev'
components = ['Library (Lib)']
creation = <Date 2014-06-18.12:41:45.841>
creator = 'Claudiu.Popa'
dependencies = []
files = ['35682', '35710']
hgrepos = []
issue_num = 21801
keywords = ['patch', 'needs review']
message_count = 5.0
messages = ['220936', '221103', '221114', '221368', '221369']
nosy_count = 5.0
nosy_names = ['ncoghlan', 'larry', 'Claudiu.Popa', 'python-dev', 'yselivanov']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue21801'
versions = ['Python 3.4', 'Python 3.5']

[PCManticore]

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.

[1st1]

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.

[PCManticore]

Your patch looks good to me.

[python-dev]

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

[1st1]

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