Thanks for the insight. That could indeed be done and I would encourage someone to come up with a fix. FWIW there is another difference between the two forms -- "from a import b" allows b to be any attribute of a, whereas "import a.b" requires b to be a submodule of a. I don't want to compromise on the latter. I do think it would be fine if "from a import b" returned the attribute 'b' of module 'a' if it exists, and otherwise look for module 'a.b' in sys.modules. On Thu, Apr 4, 2013 at 1:28 PM, PJ Eby <pje@telecommunity.com> wrote:
On Thu, Apr 4, 2013 at 11:17 AM, Guido van Rossum <guido@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.
-- --Guido van Rossum (python.org/~guido)