Description

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'

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']

Activity

1. PCManticore commented on Jun 18, 2014

   

   PCManticoremannequin

   on Jun 18, 2014

   MannequinAuthor

   More actions

   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.
2. PCManticore

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-featureA feature request or enhancementA feature request or enhancement

   on Jun 18, 2014
3. 1st1 commented on Jun 20, 2014

   

   1st1

   on Jun 20, 2014

   Member

   More actions

   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.
4. PCManticore commented on Jun 20, 2014

   

   PCManticoremannequin

   on Jun 20, 2014

   MannequinAuthor

   More actions

   Your patch looks good to me.
5. 1st1

   self-assigned this

   on Jun 20, 2014
6. python-dev commented on Jun 23, 2014

   

   python-devmannequin

   on Jun 23, 2014

   Mannequin

   More actions

   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
7. python-dev

   closed this as completedon Jun 23, 2014
8. 1st1 commented on Jun 23, 2014

   

   1st1

   on Jun 23, 2014

   Member

   More actions

   Fixed in 3.4 and 3.5.
   Thanks for the bug report!
9. ezio-melotti

   transferred this issue fromon Apr 10, 2022

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