First of all: I do know that there are already many questions and answers to the topic of the circular imports.

The answer is more or less: "Design your Module/Class structure properly and you will not need circular imports". That is true. I tried very hard to make a proper design for my current project, I in my opinion I was successful with this.

But my specific problem is the following: I need a type check in a module that is already imported by the module containing the class to check against. But this throws an import error.

Like so:

foo.py:

from bar import Bar

class Foo(object):

    def __init__(self):
        self.__bar = Bar(self)

bar.py:

from foo import Foo

class Bar(object):

    def __init__(self, arg_instance_of_foo):
        if not isinstance(arg_instance_of_foo, Foo):
            raise TypeError()

Solution 1: If I modified it to check the type by a string comparison, it will work. But I dont really like this solution (string comparsion is rather expensive for a simple type check, and could get a problem when it comes to refactoring).

bar_modified.py:

from foo import Foo

class Bar(object):

    def __init__(self, arg_instance_of_foo):
        if not arg_instance_of_foo.__class__.__name__ == "Foo":
            raise TypeError()

Solution 2: I could also pack the two classes into one module. But my project has lots of different classes like the "Bar" example, and I want to seperate them into different module files.

After my own 2 solutions are no option for me: Has anyone a nicer solution for this problem?