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Re: What's wrong with this code?

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From Andrew Cooper <amc96@cam.ac.uk>
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On 23/07/2012 15:50, Stone Li wrote:
> I'm totally confused by this code:
> 
> Code:
> 
>     a = None
>     b = None
>     c = None
>     d = None
>     x = [[a,b],
>          [c,d]]
>     e,f = x[1]
>     print e,f
>     c = 1
>     d = 2
>     print e,f
>     e = 1
>     f = 2
>     print c,d
> 
> Output:
> 
>     None None
>     None None
>     1 2
>      
> 
> 
> I'm expecting the code as:
> 
>     None None
>     1 2
>     1 2
>      
> 
> 
> What's wrong?
> And this question made my GUI program totally out of control.
> Thanks

c = 1 and d = 2 are overwriting the variable c (= None) and d (= None)
with new variables 1 and 2.  As x already captured c and d while they
were none, the variables e and f do not change (not would the, even if
you subsequently changed x)

Python is a statically scoped language, whereas the functionality you
are expecting would be an example of dynamically scoped.

Care to reveal your programming background?

~Andrew

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Thread

What's wrong with this code? Stone Li <viewfromoffice@gmail.com> - 2012-07-23 22:50 +0800
  Re: What's wrong with this code? Andrew Cooper <amc96@cam.ac.uk> - 2012-07-24 02:19 +0100
    Re: What's wrong with this code? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-07-24 07:23 +0100
    Re: What's wrong with this code? Devin Jeanpierre <jeanpierreda@gmail.com> - 2012-07-24 03:38 -0400
    Re: What's wrong with this code? Ian Kelly <ian.g.kelly@gmail.com> - 2012-07-24 02:26 -0600
  Re: What's wrong with this code? Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com> - 2012-07-24 09:47 +0200
    Re: What's wrong with this code? Chris Angelico <rosuav@gmail.com> - 2012-07-24 18:24 +1000
      Re: What's wrong with this code? Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com> - 2012-07-24 12:05 +0200
    Re: What's wrong with this code? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-07-24 08:26 +0000
    Re: What's wrong with this code? Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2012-07-24 11:34 +0200
  Re: What's wrong with this code? Thomas Rachel <nutznetz-0c1b6768-bfa9-48d5-a470-7603bd3aa915@spamschutz.glglgl.de> - 2012-07-24 10:34 +0200

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