[Python-Dev] relative import circular problem (original) (raw)

PJ Eby pje at telecommunity.com
Thu Apr 4 22:28:38 CEST 2013

On Thu, Apr 4, 2013 at 11:17 AM, Guido van Rossum <guido at python.org> wrote:

I don't really see what we could change to avoid breaking code in any particular case

Actually, the problem has nothing to do with circularity per se; it's that "import a.b" and "from a import b" behave differently in terms of how they obtain the module 'a.b'...

And "from a import b" will always fail if 'a.b' is part of a cycle with the current module, whereas "import a.b" will always succeed.

The workaround is to tell people to always use "import a.b" in the case of circular imports; it's practically a FAQ, at least to me. ;-)

The problem with "from import" is that it always tries to getattr(a,'b'), even if 'a.b' is in sys.modules. In contrast, a plain import will simply fetch a.b from sys.modules first.

In the case of a normal import, this is no problem, because a.b is set to sys.modules['a.b'] at the end of the module loading process. But if the import is circular, then the module is in sys.modules['a.b'], but not yet bound to a.b. So the "from import" fails.

So, this is actually an implementation quirk that could be fixed in a (somewhat) straightforward manner: by making "from a import b" succeed if 'a.b' is in sys.modules, the same way "import a.b" does. It would require a little bit of discussion to hash out the exact way to do it, but it could be done.

More information about the Python-Dev mailing list