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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Python tracker <[email protected]>
<http://bugs.python.org/issue24298>
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