Calling isinstance in main Python module

Question

There is a strange behavior of isinstance if used in __main__ space.

Consider the following code

a.py:

class A(object):
    pass

if __name__ == "__main__":
    from b import B
    b = B()
    print(isinstance(b, A))

b.py

from a import A
class B(A):
    pass

main.py

from a import A
from b import B

b = B()
print(isinstance(b, A))

When I run main.py, I get True, as expected, but when I run a.py, I am getting False. It looks like the name of A is getting the prefix __main__ there.

How can I get a consistent behavior? I need this trick with import of B in a.py to run doctest on file a.py.

Answer 1

WSo what happens when you run a.py is that Python reads a.py and executes it. While doing so, it imports module b, which imports module a, but it does not reuse the definitions from parsing it earlier. So now you have two copies of the definitions inside a.py, known as modules __main__ and a and thus different __main__.A and a.A.

In general you should avoid importing modules that you are executing as well. Rather you can create a new file for running doctests and use something like

import a
import doctest

doctest.testmod(a)

and remove the __main__ part from module a.