模块命名空间中提供的Python Import Statement和Recursion-need函数

Question

I have function foo() in main.py. In main.py, I import create.py. but there is a function in create.py that needs foo() from main. I can't import main.py into create.py because main.py errors out...I assume this is some kind of race condition.

How can I make foo() available in create.py namespace? It seems kind of inefficient to make foo() a module and imported by both main.py and create.py just for one function.

Answer 1

The easy answer is move foo() to foo.py and import from there or move it to create.py and import it from there into main.py - if there are things in main.py that it needs the move the too. Your other option is to pass foo from main into create as a function parameter where it is needed.

Answer 2

Just a simple hack, but this would not help in general situation.

import create.py in main.py when it is NOT called by importing.

# in main.py
if __name__ == '__main__':
    from create import *

So this will import create when you excute main.py by python main.py and that will import create and it will import main again but this time, it will see it is being imported so __name__ == '__main__' would return False . So circular chain of importing will stop.

Remember, it will not work when you will try to import main.py in some other script, because create.py won't get imported then.

So to make this thing work, you have to execute main.py, you can not import it.

Answer 3

When you import a module, Python's import machinery executes the code for that module to populate it, but only once. If you import the same module multiple times, it's fetched from the sys.modules cache instead. There's no way to end up with an infinite import loop.

However, Python adds the module to sys.modules before executing the code for it, so it won't be fully populated until the entire module has been executed. So if module A imports module B, and module B imports module A again, it will see a partially initialized module. Which is what seems to be happening here; main imports create before it's fully populated, so when create tries to access something in main , it doesn't yet exist.

Some solutions:

1. Move the shared code to a separate module, as suggested above. Imports are only done once, and Python caches compiled byte code in PYC files, so it's not really that costly.
2. Move all code into functions, and use import name everywhere (no from name import ), and call your main function from the bottom of your script. This way, all modules will finish importing before you execute anything else.
3. Move the import create statement down below the function you need access to, so it's defined before the code in create.py is executed (this is kind of silly, but can be used as a last resort).