[issue21389] The repr of BoundMethod objects sometimes incorrectly identifies the bound function

[Steven Barker]

New submission from Steven Barker:

The "repr" of bound method objects can be misleading in certain situations. The repr is always is of the format:

    <bound method x.y of <object>>

But "x" is often incorrect.

Here are some examples where the current code gets it wrong:

    # inherited method
    class Base(object):
        def foo(self):
            pass

    class Derived(Base):
        pass

    print(Derived().foo)
    # not too bad, prints "<bound method Derived.foo of <__main__.Derived object at 0xXXXX>>"
    # it probably should say "Base.foo" instead of "Derived.foo", but at least they're two names for the same function

    # however, an override and super() it gets very bad
    class Derived2(Base):
        def foo(self):
            pass

    print(super(Derived2, Derived2()).foo)
    # totally wrong, prints "<bound method Dervied2.foo of __main__.derived2 object at 0xXXXX>>"
    # but it actually *is* Base.foo bound to a Derived2 instance!

    # bound class methods:
    class Test(object):
        @classmethod
        def foo(cls):
            pass

    print(Test.foo)
    # wrong, prints <bound method type.foo of <class '__main__.Test'>>

I suggest that rather than trying to assemble the "x.y" pair by from "__self__.__class__" and "__func__.__name__", the BoundMethod should just use the "__func__.__qualname__". In each of the cases above, the function's location would be correctly located this way.

I came across this bug while investigating a confusing (to me) issue with metaclasses and inheritance. The misleading "repr" output made it much harder to figure out that my expectations were wrong. Here's a simplified example of how it led me astray:

    class A(object):
        @classmethod
        def foo(cls):
            return "classmethod from A"

    class BMeta(type):
        def foo(cls):
            return "instancemethod from BMeta"

    class B(A, metaclass=BMeta):
        pass

    print(B.foo()) # surprisingly (to me) prints "classmethod from A"
    print(B.foo)   # incorrectly prints "<bound method BMeta.foo of <class __main__.B>>"

It is presumably not a bug that inherited class methods take precedence over instance methods from a metaclass (though it was surprising to me at the time). The repr of the bound method though, suggested exactly the opposite. Given that it gets many more common situations wrong as well, I think that the repr should be fixed.

The relevant code appears to be in the method_repr function in Objects/Classobject.c .

----------
components: Interpreter Core
messages: 217566
nosy: Steven.Barker
priority: normal
severity: normal
status: open
title: The repr of BoundMethod objects sometimes incorrectly identifies the bound function
type: behavior
versions: Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21389>
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