[Python-Dev] relative import circular problem

Brett Cannon brett at python.org
Fri Apr 5 00:38:10 CEST 2013

On Thu, Apr 4, 2013 at 5:00 PM, PJ Eby <pje at telecommunity.com> wrote:

> On Thu, Apr 4, 2013 at 4:42 PM, Guido van Rossum <guido at python.org> wrote:
> > 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.
> Technically, it already does that -- but inside of __import__, not in
> the IMPORT_FROM opcode.
> But then *after* doing that check-and-fallback, __import__ doesn't
> assign a.b, because it assumes the recursive import it called has
> already done this...

It's an unfortunate side-effect of having loaders set sys.modules for new
modules not also set them as an attribute on their parent package
immediately as well (or you could argue it's a side-effect of not passing
in a module instead of a name to load_module() but that's another

>  which means that when __import__ returns, the
> IMPORT_FROM opcode tries and fails to do the getattr.
> This could be fixed in one of two ways.   Either:
> 1. Change importlib._bootstrap._handle_fromlist() to set a.b if it
> successfully imports 'a.b' (inside its duplicate handling for what
> IMPORT_FROM does), or

It's three lines, one of which is 'else:'. Just did it.

> 2. Change the IMPORT_FROM opcode to handle the fallback itself

> While the latter involves a bit of C coding, it has fewer potential
> side-effects on the import system as a whole, and simply ensures that
> if "import" would succeed, then so would "from...import" targeting the
> same module.
> (There might be other fixes I haven't thought of, but really, changing
> IMPORT_FROM to fallback to a sys.modules check is probably by far the
> least-invasive way to handle it.)

This is my preference as well. The change would be small: I think all you
need to do is if the getattr() fails then fall back to sys.modules.
Although if it were me and I was casting backwards-compatibility to the
wind I would rip out the whole fromlist part of __import__() and let the
bytecode worry about the fromlist, basically making the import opcode call
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