Test python import

a.py

class A(object):
  def __init__():
    print 'A'

b.py

from a import A

class B(A):
  def __init__():
    print 'B'

c.py

from a import A

class C(A):
  def __init__():
    print 'C'

test.py

from a import A

print A.__subclasses__()
# []

from b import B

print A.__subclasses__()
# [<class 'b.B'>]

from c import C

print A.__subclasses__()
# [<class 'b.B'>, <class 'c.C'>]

The user must cause this to be imported to indicate that they want to use it. (Perhaps directly, perhaps some runners will import certain filesystems themselves.)

Thanks