Question

foo.py :

i = 10  def fi():     global i     i = 99 

bar.py :

import foo from foo import i  print i, foo.i foo.fi() print i, foo.i 

This is problematic. Why does i not change when foo.i changes?

Answer 1

What import does in bar.py is set up an identifier called i in the bar.py module namespace that points to the same address as the identifier called i in the foo.py module namespace.

This is an important distinction… bar.i is not pointing to foo.i, but rather to the same space in memory where the object 10 is held that foo.i happens to point to at the same time. In python, the variable names are not the memory space… they are the identifier that points to a memory space. When you import in bar, you are setting up a local namespace identifier.

Your code behaves as expected until foo.fi() is called, when the identifier i in the foo.py namespace is changed to point to the literal 99, which is an object in memory obviously at a different place than 10. Now the module-level namespace dict for foo has i identifying a different object in memory than the identifier i in bar.py.

Shane and rossfabricant have good suggestions on how to adjust your modules to achieve what you want.