Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!aioe.org!feeder.news-service.com!newsfeed.xs4all.nl!newsfeed5.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
DomainKey-Signature: a=rsa-sha1; c=nofws; d=rebertia.com; s=google; h=mime-version:sender:in-reply-to:references:date :x-google-sender-auth:message-id:subject:from:to:cc:content-type :content-transfer-encoding; b=KSU9JMI2SyEKkwP4fTXoIGfNxZYqqS08HeFNzN0+NC7S3fXcru/1oAoSR5XoOhQCJU zd0kdIDbmKqD0dXR9rk0LD7isGaS/3IrGtreo/tP8HViTwGSXFtm1WsAE6rtVln3t6OK sZlzA7LRkgqj29WBZDgzNcF0sZNtUFp6cEkC0=
MIME-Version: 1.0
Sender: chris@rebertia.com
In-Reply-To: <d8d1d20e-2edd-49e5-917c-f338ef35006f@l18g2000yql.googlegroups.com>
References: <d8d1d20e-2edd-49e5-917c-f338ef35006f@l18g2000yql.googlegroups.com>
Date: Tue, 3 May 2011 09:57:53 -0700
Subject: Re: Why do directly imported variables behave differently than those attached to imported module?
From: Chris Rebert <clp2@rebertia.com>
To: Dun Peal <dunpealer@gmail.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
Cc: python-list@python.org
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.1114.1304441878.9059.python-list@python.org>
Lines: 70
NNTP-Posting-Host: 82.94.164.166
Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:4554

On Tue, May 3, 2011 at 9:31 AM, Dun Peal <dunpealer@gmail.com> wrote:
> Hi!
>
> Here's the demonstrating code:
>
> =C2=A0 =C2=A0# module foo.py
> =C2=A0 =C2=A0var =3D 0
>
> =C2=A0 =C2=A0def set():
> =C2=A0 =C2=A0 =C2=A0 =C2=A0global var
> =C2=A0 =C2=A0 =C2=A0 =C2=A0var =3D 1
>
> Script using this module:
>
> =C2=A0 =C2=A0import foo
> =C2=A0 =C2=A0from foo import *
>
> =C2=A0 =C2=A0print var, foo.var
> =C2=A0 =C2=A0set()
> =C2=A0 =C2=A0print var, foo.var
>
> Script output:
>
> =C2=A0 =C2=A00 0
> =C2=A0 =C2=A00 1
>
> Apparently, the `var` we imported from `foo` never got set, but
> `foo.var` on the imported `foo` - did. Why?

Because imports (and assignments generally) bind names to values, they
don't alias names to other names.

from foo import *

can be thought of as essentially doing:

import foo
set =3D foo.set
var =3D foo.var
del foo

So the new, separate name __main__.var gets the current value of
foo.var at import-time, which is the integer 0.
You then call foo.set(), which re-binds foo.var to a new value (i.e.
1) rather than mutating the existing value (which would be impossible
anyway since integers are immutable). This has absolutely no effect on
__main__.var, which is an entirely separate binding.

The behavior is comparable to that of function arguments. Values can
be mutated, but re-binding names has only local effect:
>>> a =3D 0
>>> def incr(b):
...     b =3D 1 # rebinds local name b
...
>>> incr(a)
>>> a # outside name unaffected, just like in your example
0
>>> c =3D [7]
>>> def mutate_then_rebind(b):
...     b.append(99) # mutates passed-in value
...     b =3D [42] # rebinds local name; has no outside effect
...
>>> mutate_then_rebind(c)
>>> c # name still references same obj, but that obj has been mutated
[7, 99]

Cheers,
Chris
--
http://rebertia.com