why the value of `*args` is strange?

Question

Here is my code snappt:

class C(dict):
    def __init__(*args, **kwds):
        print 'args = {}'.format(args)
        print 'kwds = {}'.format(kwds)
        if not args:
            raise TypeError("descriptor '__init__' of 'Counter' object needs an argument")

        self = args[0]

        args = args[1:]

        if len(args) > 1:
            raise TypeError('expected at most 1 arguments, got %d' % len(args))

        super(C, self).__init__()


if __name__ == '__main__':
    c = C()

When i run this code, i am confused with the output:

C:\Users\elqstux\Desktop>python wy.py
args = ({},)
kwds = {}

For c = C(), i think the code will raise a Error, because args will be () in this case, but from the output, args is ({},), what't the reason?

self, args = args[0], args[1:] # allow the "self" keyword be passed

Answer 1

*args is ({},) because you didn't declare an explicit self parameter. So when __init__ is called, the self parameter ends up becoming the first element in args. Since you inherit from dict, you get dict's repr, which is {}.

This trick is used intentionally in Python's dict subclasses nowadays, because otherwise code that initialized the dict subclass with mydict(self=123) would break the initializer. The fix is to do:

 def __init__(*args, **kwargs):
     self, *args = args
     # On Python 2 without unpacking generalizations you'd use the more repetitive:
     # self, args = args[0], args[1:]

which forces self to be accepted purely as a positional argument (which is passed implicitly by the instance construction machinery). For an example of real world usage, see the implementation of collections.Counter.