Good point. Consider

    >>> def f(*argv): pass
    >>> d = dict()

Now compare

    >>> f(1, 2) = 3
    SyntaxError: can't assign to function call
    >>> d[1, 2] = 3

    >>> d[1, 2]

    3

Item assignment (ie __setitem__) is the one thing that a function call can't do. If we want keywords in our __getitem__ and so on commands, then one route for item assignment is to allow

    >>> d[1, 2, a=3, b=4] = 5

as valid syntax.

By the way, another route is to use a simple function call, like so

    >>> d[o(1, 2, a=3, b=4)] = 5

which is already possible today. Some of us don't like this route.

-- 

Jonathan