[Tutor] global list

Gerardo Juárez gerardojuarez at buyteknet.info
Thu Apr 24 02:26:02 CEST 2014

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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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

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:


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 


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