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I have a question on a fairly simple task in python, however, I didn't manage to find a solution. I would like to assign a new value to an already existing variable in python. However, the changes I am doing to the variable within the function don't stick to the variable.

Here is a simplified example of my problem:

y = 1
x = None

def test(var):

    var = y

    return var 


test(x)
print(x)

The print simply returns none. So the changes I have done to the variable within the function are non permanent.

How can I make the changes on the input-variable of the function permanent?

Thanks in advance!

ZeroStrikeCall
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2 Answers2

7

Variables in Python are just names which refer to objects. In an expression, the name is a stand-in for the actual object. Saying test(x) means "pass the object referred to by x into test". It does not mean "pass the symbol x into test".

In addition, re-assigning a name only changes what object that name refers to. It affects neither the object nor any of its aliases.

In short, the name var you modify inside test has no relation to x at all.

The preferred way to have a function change something is by reassigning the result:

x = 2
def change(var):
    return var * 2

x = change(x)  # x now refers to 4 instead of 2
print(x)

If you want to change a name outside a function, you can use the nonlocal and global keywords:

x = 2
def change_x():
    global x
    x = x * 2

change_x()  # x now refers to 4 instead of 2
print(x)

While this can make some trivial problems easy to solve, it is generally a bad idea for larger programs. Using global variables means one can no longer use the function in isolation; results may depend on how often and in what order such a function is called.

If you have some self-contained group of values and means to modify them, a class can be used to describe this:

class XY:
    def __init__(self, x, y):
        self.x, self.y = x, y

    def swap(self):
        self.x, self.y = self.y, self.x

my_values = XY(None, 1)
print(my_values.x, my_values.y)
my_values.swap()
print(my_values.x, my_values.y)

In contrast to global variables, you can create as many isolated instances of classes as needed. Each instance can be worked on in isolation, without affecting the others.

You can also use mutable values to make changes visible to the outside. Instead of changing the name, you modify the value.

x = [2]  # x is a mutable list, containing the value 2 at the moment
def change(var):
    var[0] = 4  # change leading element of argument

change(x)  # x now contains 4 instead of 2
print(x)
MisterMiyagi
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2

This is an example of passing variables to functions by value. By default, when you pass a variable to a function in Python, it is passed by value.

What it means is, that a new variable with a new scope is created with the same value of x. So, any change that happens to the new x is not reflected to the x outside the function's scope.

If you want to get the value from the function back, you can use the return statement (as you have used). return returns the value back from the function. However, in your example there is no variable to receive it. Hence, it is lost.

You would have to call the function as x = test(x). This ensures that x receives the value back from the function.

MaJoR
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