Martijn Pieters added the comment:

A temporary work-around is to use a function to raise a NameError exception 
when the module attribute doesn't exist yet:

def _forward_A_reference():
    try:
        return a.A
    except AttributeError:
        # not yet..
        raise NameError('A')

class B:
    def spam(self: 'B', eggs: typing.Union['_forward_A_reference()', None]):
        pass

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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue26477>
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