Why can function definitions only use identifiers, and not attribute references or any other primaries?

[Jeremy Banks]

Hi. I'm sure there've been debates about this before, but I can't seem
to figure out what to search for to pull them up, so I'm asking here.

It seems to me that a lot of things could be made much easier if you
could use primaries other than basic identifiers for the target of
function definitions. For example, if attribute references were
allowed I'd be able to do:

    def foo.bar():
        return("I'm a method!")

If we wanted to get even more liberal (which I don't see as a bad
thing, but I could see being found more objectionable by some), we
could allow the use of anything that's a valid assignment target. For
example:

    def foo["bar']():
        return("I'm a function referenced in a mapping object!")


In this case I could see there being a problem in that there's nothing
to get the function's __name__ from, but that doesn't apply for the
first example.

Many uses of this may not be Pythonic, but I'm sure there are many
that are. It just feels like an arbitrary restriction, preventing
users from doing something that may be useful.

Any feedback or direction to previous discussion on the subject would
be appreciated. Thanks!