from xx import yy

[Chris Angelico]

On Mon, Nov 13, 2017 at 1:17 PM, bvdp <bob at mellowood.ca> wrote:
> I'm having a conceptual mind-fart today. I just modified a bunch of code to use "from xx import variable" when variable is a global in xx.py. But, when I change/read 'variable' it doesn't appear to change. I've written a bit of code to show the problem:
>
> mod1.py
> myvar = 99
> def setvar(x):
>     global myvar
>     myvar = x
>
> test1.py
> import mod1
> mod1.myvar = 44
> print (mod1.myvar)
> mod1.setvar(33)
> print (mod1.myvar)
>
> If this test1.py is run myvar is fine. But, if I run:
>
> test2.py
> from mod1 import myvar, setvar
> myvar = 44
> print (myvar)
> setvar(33)
> print (myvar)
>
> It doesn't print the '33'.
>
> I thought (apparently incorrectly) that import as would import the name myvar into the current module's namespace where it could be read by functions in the module????

It imports the *value*. What you have is basically this:

import mod1
myvar = mod1.myvar
setvar = mod1.setvar

Since functions remember their contexts, setvar() still sees the
globals of mod1. But myvar is a simple integer, so it's basically
"myvar = 99".

So basically, you can't from-import anything that's going to be
changed. You can import constants that way ("from stat import
S_IREAD"), or classes/functions (since they're not generally rebound),
but as a general rule, don't from-import anything mutable. In fact, if
you follow the even-more-general rule of "don't bother with
from-imports at all", you'll be right far more than you'll be wrong.

ChrisA