> And if os.open were supplied in os.py:
>
>     @inspect.override_string_representation('mode',
>         'os.O_CREAT | os.O_RDWR')
>     def open(file, flags, mode=0o777, *, dir_fd=None):

Other use case is a sentinel default. "foo(arg={})" looks better than "foo(arg=<object object>)" for function which use a sentinel idiom:

    _sentinel = object()
    def foo(arg=_sentinel):
        if arg is _sentinel:
            arg = {}
        ...

Sometimes full signature overwriting is needed (see for example Python implementation of operator.methodcaller() in issue16694).