Variable scope (Python Newbie)

Question

I am using a .pop method on a Global list inside a function block, but the Global list is being updated outside the block. I thought that local variables can't modify Global variables.

This should not work, but it does:

import random

PhraseBank = ['a','b','c','d']

def getPuzzle(secretphrase):
    phraseIndex = random.randint(0,len(PhraseBank)-1)
    secretphrase = PhraseBank.pop(phraseIndex)
    return secretphrase #Returns and item indexed from the PhraseBank

while len(PhraseBank) != 0:
    secretphrase = getPuzzle(PhraseBank) #works
    print(secretphrase, PhraseBank)

OUTPUT is:

a ['b', 'c', 'd']
d ['b', 'c']
c ['b']
b []

Why is PhraseBank getting updated Globally when I am only modifying it inside a function block?

The PhraseBank list is not copied when passed to getPuzzle(); names are local, but names are just references. — Martijn Pieters
– Martijn Pieters, Commented Feb 28, 2014 at 14:22

RemcoGerlich · Accepted Answer · 2014-02-28 14:21:25Z

4

Lists are mutable. You are changing the list that PhraseBank refers to, but it's still referring to the same list. So the variable isn't changed (still refers to the same thing) but that thing has changed.

answered Feb 28, 2014 at 14:21

RemcoGerlich

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Ellioh · Accepted Answer · 2014-02-28 14:36:46Z

3

If you were assigning a value to the PhraseBank, it would not be changed unless you say you are using the global variable explicitly. However, each var in Python is just a named reference to an object. You read the var and then modify the object it refers to. Yes, you can't change the reference itself, but you can change the object.

So, what you faced is one of the most typical features of Python. Everything is an object, all the variables are references. Understanding that fact often helps to understand many things that may seem strange. A good example:

>>> li = [1]
>>> a = (li, li)
>>> a[0].append(1)
>>> a[1]
[1, 1]
>>> li
[1, 1]
>>> li.append(1)
>>> a
([1, 1, 1], [1, 1, 1])

If nothing surprises you in the behavior of the code above, then you understand how are the variables and objects related. :-) Here the variables that are not touched are changed, but not because they start referring other objects, but because the objects they refer to are modified. So do the tuples that are immutable. Yes, they are. A tuple, once created, always refers to the same objects. But each of those object may be changed.

edited Feb 28, 2014 at 14:36

answered Feb 28, 2014 at 14:22

Ellioh

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1 Comment

theBIOguy Over a year ago

I actually DO understand the code above! But I couldn't figure out the behavior inside and outside a block. But your explanation is a HUGE help. Thanks, I got it now!

user2357112 · Accepted Answer · 2014-02-28 14:22:18Z

2

You can't assign to a global variable inside a function (unless you explicitly use a global declaration). You can modify objects stored in global variables just fine.

answered Feb 28, 2014 at 14:22

user2357112

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