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Python stdlib code that looks odd

I'm poking around in the stdlib, and Lib/mailbox.py has the following
inside class Mailbox:

    def update(self, arg=None):
        """Change the messages that correspond to certain keys."""
        if hasattr(arg, 'iteritems'):
            source = arg.items()
        elif hasattr(arg, 'items'):
            source = arg.items()
            source = arg
        bad_key = False
        for key, message in source:
            ... use key/message ...

Looks odd to check if it has iteritems and then use items. Presumably
this will catch a Python 2 dictionary, and take its items as a list
rather than an iterator; but since the only thing it does with source
is iterate over it, would it be better done as iteritems? Mainly, it
just looks really weird to check for one thing and then call on
another, especially as it then checks for the other thing in the next