1

I read this post about how to prevent circular import in python. I don't understand a claim in the post: 

import package.a # Absolute import and 
import a # Implicit relative import (deprecated, py2 only)

can avoid circular import but

from ... import ...
#or
import ... as ..

can't (on python 2.x).

Does anyone know the reason behind this?

Community
  • 1
  • 1
del bao
  • 1,084
  • 1
  • 11
  • 20

1 Answers1

0

After some search, I figure out the answer myself.

Essentially, circular import is a problem for from … import … because it returns the imported module only after the module code is executed.

To illustrate, assume we have import a in b.py and import b in a.py. For import a and import b, I simply look up sys.modules to find a/b, just put a new module in and return.

On the other hand, from a import c in b.py has code like this (pseudo python, similar for from b import d in a.py)

if 'a' not in sys.modules:
    sys.modules[a] = (A new empty module object)
    run every line of code in a.py
    add the module a to its parent's namespace
return module 'a'

we start from a.py, put an empty module for b to sys.modules and start executing b.py and put a in sys.modules and executing a.py. Next it hits from b import d again and find the b in sys.modules but it's empty, throws error: no attribute d.

PS1: the referred post is wrong. import ... as ... is good for circular import.

PS2: from a import c is the same to import a.c in python 3

del bao
  • 1,084
  • 1
  • 11
  • 20