instantiate all subclasses of a class

[Daniel Nogradi]

What is the simplest way to instantiate all classes that are
subclasses of a given class in a module?

More precisely I have a module m with some content:

# m.py
class A:
    pass
class x( A ):
    pass
class y( A ):
    pass
# all kinds of other objects follow
# end of m.py

and then in another module I have currently:

# n.py
import m
x = m.x( )
y = m.y( )
# end of n.py

and would like to automate this in a way that results in having
instances of classes from m in n whose names are the same as the
classes themselves. But I only would like to do this with classes that
are subclasses of A.

Any ideas?