Okay, I do understand that this topic is old as hell, but I couldn't find an answer to the particular question that I am asking.

Let's say that we have a very simple structure: two files, a.py and b.py, their contents being:

a.py

import bclass C:lal = 4class A:kek = 12lol = b.B()

b.py

import aclass B:aa = a.C()

Trying to run python b.py, we get:

Traceback (most recent call last):File "b.py", line 1, in <module>import aFile ".../a.py", line 1, in <module>import bFile ".../b.py", line 3, in <module>class B:File ".../a.py", line 5, in A                                                                                  aa = a.C()
AttributeError: module 'a' has no attribute 'C'

BUT if we move the import b line AFTER the C class, the script launches and produces no errors.

I have not found any mention of this whatsoever in any answers here on SO. The question here is: Why does this happen and how to escape this?

This is a particularly important question for the Django framework. When I have many models, I try splitting them into many files. It is very easy to get a cyclic import there.

In python when you import a module then it first import all module that defines at the top of the module, If a module is not in sys.modules, then an import creates the new module entry in sys.modules and then executes the code in the module.

so when you try to import b.py module in a.py then it first import all module that listed in b.py is a.py (import a) if module not listed in sys.modules . and still module b.py not completely executed so b.py module is not added into sys.modules

after that, it tries to import a.py and in a.py it tries first import all module that import in a.py is b.py

so it's a basic cycle a.py try to import b.py and b.py try to import a.py

For that problem, Solution is import one module or class inside the class, Not at the top of the module

as per your example

a.py

class C:lal = 4class A:import bkek = 12lol = b.B()

b.py

import aclass B:aa = a.C() 

or

a.py

import b
class C:lal = 4class A:kek = 12lol = b.B()

b.py

class B:import aaa = a.C() 

for detailed information discussion

Python issue