[Tutor] global list

[Gerardo Juárez]

On 04/23/2014 06:46 PM, Denis Heidtmann wrote:
> In a coursera python course video the following code was presented:
>
> a = [4,5,6]
>
> def mutate_part(x):
>      a[1] = x
>
> mutate_part(200)
>
> The presenter said something like "a is a global variable, so a becomes
>
> [4,200,6] after running mutate_part(200)."
>
> Indeed it does, but why does this work without specifying a as global
> within mutate()?
> My thinking was that an "undefined" error should have been raised.
>
> Help me understand.  Thanks,
>
> -Denis
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Hi,

Try out this modified version of the example you provide:


def mutate(x):
     global y
     y = x + 1
     a[1] = x

a = [1, 2, 3]
y = 0

mutate(200)
print a
print y

Both, 'a' and 'y' will be modified, but if you comment out global, 'y' 
will not change.
In a few words, assigning a[k] = value is technically a method call, and 
it behaves
differently (spans across scopes). A more detailed answer can be found here:

http://stackoverflow.com/questions/4630543/defining-lists-as-global-variable-in-python

Of course, I would limit this practice to *very* small programs, since 
anything beyond
50 lines should have more structure and you can use either modular 
programming or
OOP, which is the preferred practice today to structure complex 
applications.

Gerardo