__subclasses__ changes when new classes come into scope via an import

gistfile1.py

# crap/a.py

class A(object):
    pass

print A.__subclasses__() # this will be an empty list, []

# crap/b.py

from a import A

class B(A):
    pass
    
print A.__subclasses__() # this will now contain B, [<class '__main__.B'>]

# If another module imports a but not b, A.__subclasses__() will be an empty list, even if module b is in the path