Python: How do I avoid my global variable from being updated when passing it through a function?

Question

Below is a simplified version of my problem:

example =[
    [0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0]
    ]

def func(data):
    data[0][0:6] = [1, 1, 1, 1, 1]
    data[1][0:6] = [1, 1, 1, 1, 1]
    return data    

print(example)
func(example)
print(example)

and has the output:

[[0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]
[[1, 1, 1, 1, 1], [1, 1, 1, 1, 1]]

The output I am expecting is:

[[0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]
[[0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]

How do I avoid the global variable "example" from being updated after running "func"? I have tried a number placeholder variable combinations (inside and outside "func") and all have the same result - an updated "example".

It looks like you could use a [quick guide](https://nedbatchelder.com/text/names.html) to the relationship between variables and objects in Python. Python doesn't implicitly copy objects all over the place like, say, C++. — user2357112, May 27 '20 at 21:36
Try using shallow copy - pass `copy.copy(example)` to the function. — DisappointedByUnaccountableMod, May 27 '20 at 21:36
Also it’s not just a global variable that could be affected like this - it’s how Python works. — DisappointedByUnaccountableMod, May 27 '20 at 21:37
Does this answer your question? [How to clone or copy a list?](https://stackoverflow.com/questions/2612802/how-to-clone-or-copy-a-list) — mkrieger1, May 27 '20 at 21:38

score 1 · Answer 1 · answered May 27 '20 at 21:37

While you should avoid using globals, if you absolutely need to do the above, use deepcopy():

from copy import deepcopy

example =[
    [0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0]
    ]

def func(data):
    data = deepcopy(data)
    data[0][0:6] = [1, 1, 1, 1, 1]
    data[1][0:6] = [1, 1, 1, 1, 1]
    return data

print(example)
func(example)
print(example)

score 0 · Answer 2 · answered May 27 '20 at 21:37

Try using deep copy:

import copy
example =[
    [0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0]
    ]

def func(data):
    data[0][0:6] = [1, 1, 1, 1, 1]
    data[1][0:6] = [1, 1, 1, 1, 1]
    return data    

print(example)
func(copy.deepcopy(example))
print(example)

score 0 · Answer 3 · answered May 27 '20 at 21:42

Because lists are mutable, they are basically passed by reference to func. If you want to act on a different copy of that object, you need to explicitly make such a copy (other answers say the same, but for your case you probably want that inside the function):

import copy

example =[
    [0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0]
    ]

def func(data):
    localdata = copy.deepcopy(data). ## or data.copy() or copy.copy(data)
    localdata[0][0:6] = [1, 1, 1, 1, 1]
    localdata[1][0:6] = [1, 1, 1, 1, 1]
    return localdata    

print(example)
newexample = func(example)
print(example)

Whether you want copy.copy(data) or copy.deepcopy(data) or data.copy() depends on what you know about the input.

Note also that your original code doesn't use its return for anything...

Python argument passing mechanics don't change depending on whether the argument is mutable. (If it looks like they do, that's because of the operations you're testing. Comparisons like `x = 3` vs `y[0] = 4` are misleading - `y[0] = 4` isn't an assignment to `y`.) — user2357112, May 27 '20 at 22:23
@user2357112supportsMonica That is correct, of course, but I didn't feel it was particularly helpful to go into the semantics of Python name binding which underlies all of this. — Andrew Jaffe, May 27 '20 at 22:29

score 0 · Answer 4 · answered May 27 '20 at 21:43

You need to make a copy for this to work. And since example is a list of lists, you have to either use copy.deepcopy() or list comprehensions, like the following:

example =[
    [0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0]
    ]

def func(data):
    dataCopy = [sublist.copy() for sublist in data]
    dataCopy[0][0:6] = [1, 1, 1, 1, 1]
    dataCopy[1][0:6] = [1, 1, 1, 1, 1]
    return dataCopy    

print(example) # all 0's
func(example)
print(example) # still all 0's

print(func(example)) # all 1's

Python: How do I avoid my global variable from being updated when passing it through a function?

4 Answers4