Mailing List Archive: A small module question

I am having a problem, which is reducible to the following, which I
would be happy if someone could explain to me:

[redhog@k35 Python]$ ls -R test/
test:
__init__.py a.py b.py
[redhog@k35 Python]$ cat test/__init__.py
__all__ = ["a", "b"]
[redhog@k35 Python]$ cat test/a.py
from test import b
[redhog@k35 Python]$ cat test/b.py
from test import a
[redhog@k35 Python]$ python
Python 1.5.1 (#1, Mar 21 1999, 22:49:36) [GCC egcs-2.91.66 19990314/Li
on linux-i386
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> from test import a
Traceback (innermost last):
File "<stdin>", line 1, in ?
File "test/a.py", line 1, in ?
from test import b
File "test/b.py", line 1, in ?
from test import a
ImportError: cannot import name a
>>>

redhog@lysator.liu.se writes:

> I am having a problem, which is reducible to the following, which I
> would be happy if someone could explain to me:
>
> [redhog@k35 Python]$ ls -R test/
> test:
> __init__.py a.py b.py
> [redhog@k35 Python]$ cat test/__init__.py
> __all__ = ["a", "b"]
> [redhog@k35 Python]$ cat test/a.py
> from test import b
> [redhog@k35 Python]$ cat test/b.py
> from test import a
> [redhog@k35 Python]$ python
> Python 1.5.1 (#1, Mar 21 1999, 22:49:36) [GCC egcs-2.91.66 19990314/Li
> on linux-i386
> Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
> >>> from test import a
> Traceback (innermost last):
> File "<stdin>", line 1, in ?
> File "test/a.py", line 1, in ?
> from test import b
> File "test/b.py", line 1, in ?
> from test import a
> ImportError: cannot import name a
> >>>

The "from test import a" statement ends with referencing test.a while
a isn't completely imported. The key "test.a" exists in sys.modules,
and "import test.a" would work; however the attribute a isn't added to
the module test until after the import of a is completed.

So the solution is to use "import a" and "import b" instead -- inside
the package test, this will refer to modules test.a and test.b anyway,
if they exist in the package.

--Guido van Rossum (home page: http://www.python.org/~guido/)