Python: isinstance doesn't work properly?

Question

I've got a problem with isinstance and I've reduced it to this case.

I have two files.

main.py:

import testType
class TestClass:
    def __init__(self):
        pass
if __name__ == '__main__':
    test_class = TestClass()
    testType.test_type(test_class)

and testType.py

from main import TestClass
def test_type(obj):
    print(type(obj), TestClass)
    print(isinstance(obj, TestClass))

And I've got:

<class '__main__.TestClass'> <class 'main.TestClass'>
False

Why?

How should I fix it?