[issue24298] inspect.signature includes bound argument for wrappers around bound methods

[Yury Selivanov]

Changes by Yury Selivanov :


--
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue24298] inspect.signature includes bound argument for wrappers around bound methods

[Roundup Robot]

Roundup Robot added the comment:

New changeset 42819b176d63 by Yury Selivanov in branch '3.4':
Issue 24298: Fix signature() to properly unwrap wrappers around bound methods
https://hg.python.org/cpython/rev/42819b176d63

New changeset d7e392c5c53a by Yury Selivanov in branch '3.5':
Issue 24298: Fix signature() to properly unwrap wrappers around bound methods
https://hg.python.org/cpython/rev/d7e392c5c53a

New changeset ab46801ca359 by Yury Selivanov in branch 'default':
Issue 24298: Fix signature() to properly unwrap wrappers around bound methods
https://hg.python.org/cpython/rev/ab46801ca359

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nosy: +python-dev

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[issue24298] inspect.signature includes bound argument for wrappers around bound methods

[Yury Selivanov]

Yury Selivanov added the comment:

> I'm OK with the patch as is, but I'm definitely concerned about the 
> maintainability of inspect.signature in general.

I agree, _signature_from_callable is getting quite complex.  The good news is 
that we have a very good test coverage for signatures.

As for a big block comment -- it might be useful.  OTOH the code of 
_signature_from_callable is mostly if..else statements with calls to helpers 
and recursion.

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[issue24298] inspect.signature includes bound argument for wrappers around bound methods

[Nick Coghlan]

Nick Coghlan added the comment:

I'm OK with the patch as is, but I'm definitely concerned about the 
maintainability of inspect.signature in general.

I'm trying to decide if a block comment covering the order of calling protocols 
that we check, and where we potentially recurse, would be a help (by providing 
a map of the function for the benefit of future maintainers) or a hindrance (by 
providing the opportunity for that map to get out of sync)

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[issue24298] inspect.signature includes bound argument for wrappers around bound methods

[Yury Selivanov]

Yury Selivanov added the comment:

> That's also a little more future-proof, in case any further checks happen to 
> be inserted ahead of the check for __wrapped__.

Well, we unwrap until we see a "__signature__" attribute (or we can't unwrap 
anymore).  And right after unwrapping we try to return the __signature__; so 
inserting more checks before unwrapping with unconditional recursion after it 
won't be so safe.

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[issue24298] inspect.signature includes bound argument for wrappers around bound methods

[Nick Coghlan]

Nick Coghlan added the comment:

It occurs to me we're also bypassing the check that the unwrapped obj is a 
callable, so it probably makes sense to just recurse unconditionally after 
unwrapping the object, rather than only recursing for methods.

That's also a little more future-proof, in case any further checks happen to be 
inserted ahead of the check for __wrapped__.

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[issue24298] inspect.signature includes bound argument for wrappers around bound methods

[Yury Selivanov]

Changes by Yury Selivanov :


Added file: http://bugs.python.org/file39526/signature2.patch

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[issue24298] inspect.signature includes bound argument for wrappers around bound methods

[Yury Selivanov]

Yury Selivanov added the comment:

Thanks for reporting this, Petr!

Nick, could you please take a look at the patch?

--
assignee:  -> yselivanov
keywords: +patch
stage:  -> patch review
versions: +Python 3.6
Added file: http://bugs.python.org/file39524/signature.patch

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[issue24298] inspect.signature includes bound argument for wrappers around bound methods

[Yury Selivanov]

Changes by Yury Selivanov :


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nosy: +ncoghlan

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[issue24298] inspect.signature includes bound argument for wrappers around bound methods

[Yury Selivanov]

Changes by Yury Selivanov :


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nosy: +yselivanov

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[issue24298] inspect.signature includes bound argument for wrappers around bound methods

[Petr Viktorin]

New submission from Petr Viktorin:

When obtaining the signature of a bound method, inspect.signature, by default, 
omits the "self" argument to the method, since it is already specified in the 
bound method.  However, if you create a wrapper around a bound method with 
functools.update_wrapper() or @functools.wraps, calling inspect.signature on 
the wrapper will return a signature which includes the "self" argument.

Reproducer:

import inspect
import functools

class Foo(object):
def bar(self, testarg):
pass

f = Foo()

@functools.wraps(f.bar)
def baz(*args):
f.bar(*args)


assert inspect.signature(baz) == inspect.signature(f.bar)

The program will fail with an assertion error. Examining inspect.signature(baz) 
shows:

>>> print(inspect.signature(baz))
(self, testarg)
>>> print(inspect.signature(f.bar))
(testarg)

Looking at the code in inspect.py:

The handling of bound methods appears at the top of 
inspect._signature_internal().  Since baz is not itself a bound method, it 
doesn't trigger this case.  Instead inspect.unwrap is called, returning f.bar.

inspect._signature_is_functionlike(f.bar) returns True, causing 
Signature.from_function to be called.  Unlike the direct bound method case, 
this includes the bound method's "self" argument.

--
messages: 244178
nosy: encukou
priority: normal
severity: normal
status: open
title: inspect.signature includes bound argument for wrappers around bound 
methods
versions: Python 3.4, Python 3.5

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[issue24298] inspect.signature includes bound argument for wrappers around bound methods

[Petr Viktorin]

Petr Viktorin added the comment:

Reported by David Gibson here: 
https://bugzilla.redhat.com/show_bug.cgi?id=1201990

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