Suppose you have the following modules:

foo.py:

from bar import bar_var
foo_var=1

bar.py:

from foo import foo_var
bar_var=2

The problem is that the interpreter will perform the following steps:

• main imports foo
• Empty globals for foo are created
• foo is compiled and starts executing
• foo imports bar
• Empty globals for bar are created
• bar is compiled and starts executing
• bar imports foo (which is a no-op since there already is a module named foo)
• bar.foo_var = foo.foo_var

The last step fails, because Python isn't done with interpreting foo yet and the global symbol dictionary for foo is still empty.

The same thing happens when you use import foo, and then try to access foo.foo_var in global code.

There are (at least) three possible workarounds for this problem.

Guido van Rossum recommends avoiding all uses of from <module> import ..., and placing all code inside functions. Initializations of global variables and class variables should use constants or built-in functions only. This means everything from an imported module is referenced as <module>.<name>.

Jim Roskind suggests performing steps in the following order in each module:

• exports (globals, functions, and classes that don't need imported base classes)
• import statements
• active code (including globals that are initialized from imported values).

van Rossum doesn't like this approach much because the imports appear in a strange place, but it does work.

Matthias Urlichs recommends restructuring your code so that the recursive import is not necessary in the first place.

These solutions are not mutually exclusive.

CATEGORY: programming