[Python-Dev] [Python-checkins] r43033 - in python/trunk/Lib: distutils/sysconfig.py encodings/__init__.py
Phillip J. Eby
pje at telecommunity.com
Wed Mar 15 23:18:43 CET 2006
At 01:34 PM 3/15/2006 -0800, Guido van Rossum wrote:
>Because Thomas designed it this way. :-)
>
>I believe his design makes sense though: "import foo" translates to
>__import__(foo, ...).
>
>There's a separate setting, only known to the compiler, that says
>whether "from __future__ import absolute_import" is in effect (there's
>no way to slip a flag into globals to convey this setting, since code
>is compiled independently from globals, and there are ways to pass
>flags to the compiler without explicitly doing the future import).
>
>So the compiler emits different code when the future syntax is in
>effect, and that opcode must pass its knowledge to __import__. This is
>done trough the 5th argument, which also tells us how many leading
>dots there were. Without the future import, the 5th argument is
>omitted (as long as there aren't any leading dots).
Ah, so it's *relative* imports that require a 5th argument. I was thinking
it was there to support absolute imports. I was thinking that relative
imports could be implemented by popping bits off of __name__ to get an
absolute location.
It seems to me that backward compatibility would be greatly enhanced by
having the interpreter convert everything but "legacy" imports into
absolute imports, as this would then work with the existing __import__ code
in the field, even when new relative/absolute code was doing the
importing. Otherwise, this forces __import__ hooks to be rewritten. (I
personally avoid writing __import__ hooks if at all possible, but there are
certainly some out there.)
The mechanism I have in mind would be to just have an IMPORT_EXACT opcode
that takes a relative or absolute name. This opcode would process relative
names relative to the __name__ in globals to produce an exact module name,
and leave absolute names alone. It would then invoke __import__ using the
builtins or sys module dictionary as the "globals" argument *instead of the
current globals*, so that __import__ will not do any legacy-style fallback.
When absolute imports are in effect, or when an explicit relative import is
used, it would be compiled such that IMPORT_NAME is replaced by IMPORT_EXACT.
This mechanism doesn't require any change to the __import__() signature,
and so remains backward compatible with any existing import hook that
doesn't do weird things to the globals dictionary of the module that
invoked it.
On the other hand, perhaps it would be better to fail loudly by breaking on
the 5th argument, than to fail silently for really weird __import__
hooks. That is, if it breaks, it will force people to make sure their
__import__ code is safe for use with absolute imports. So, the existing
approach might well be better than what I had in mind.
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