Python function changing global variable?

Question

So I have a question that pertains to the scope of variables in functions in python, consider the following code:

import numpy as np


def function(data, mask):
    data *= mask


data = np.ones(5)
for x in range(2):
    print(data)
    mask = np.array([1, 0, 0, 0, 0])
    function(data, mask)

And the output is the following:

[1. 1. 1. 1. 1.]
[1. 0. 0. 0. 0.]

My question is why data changes after the first iteration? I always thought that the local variable of a function was... local, so any modification would remain confined within the scope of the function. Clearly I was wrong. Is this because data in the function is still referencing the same object as the data in the global scope?

CrisPlusPlus · Accepted Answer · 2022-10-21 09:53:52Z

2

In Python, mutable objects passed to a function will mutate in-place if requested. Specifically, you are telling function to mutate data in-place by using the operator *=. If, on the other hand, you don't want data to mutate, simply do

data = data * mask

which will simply assign the local (i.e. within the function scope) tag data to a local variable.

answered Oct 21, 2022 at 9:53

CrisPlusPlus

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Thomas · Accepted Answer · 2022-10-21 09:54:25Z

2

The data in the function can be a different one than the global data, because it's passed as a parameter. The name is the same, so it shadows the global, but they are otherwise unrelated...

... until you call function(data, mask), passing the global data in. The function modifies the local data, but because it refers to the same object as the global one, the change is visible outside the function.

answered Oct 21, 2022 at 9:54

Thomas

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smyril · Accepted Answer · 2022-10-21 09:57:11Z

1

I would suggest you to make a copy of your variable:

def function(data)
    dataf = data.copy()

I would also suggest to use numpy.multiply instead of *=:

data = numpy.multiply(data, mask)

answered Oct 21, 2022 at 9:57

smyril

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