How to modify a list within a Python method?

Question

Suppose I have the following method and the following list:

def ModificaValores(Valores):
      dados = [6,5,4,3,2,1,0]

      Valores = dados

Valores = [0,1,2,3,4,5,6]

print(Valores)
ModificaValores(Valores)
print(Valores)

Because on the way out I have the following result:

[0, 1, 2, 3, 4, 5, 6]
[0, 1, 2, 3, 4, 5, 6]

If I modify element by element it works, but not like this. Explain to me why it happens!

Answer 1

It doesn’t work because the changes made within the scope of the function are not loaded into the external scope - nor should it, as this could generate side effects and make it very difficult to maintain the code. I mean, don’t do it unless it actually makes sense.

Let’s go in pieces.

Outside the function, in the global scope, you define the list Valores. At this time, the object Valores shall have a unique identifier representing it that can be obtained via id(Valores). In Cpython, this value represents the memory address of your object. This object is passed via parameter to the function ModificaValores and, as in Python the type list is a changeable type, the object will be passed by reference, which implies that within the function will be the same object that was created outside, with the same values and same memory address. The fact that it is passed via reference explains why it reflects the changes made element by element, as you are only modifying the object. Even if this reflects the changes to the external scope, it should be used with great caution, only in cases that really make sense. Overall, the ideal would be to return a new list and assign it to the desired object. Something like:

def ModificaValores(Valores):
    # Neste caso o parâmetro é desnecessário
    # mas mantive por questões didáticas
    dados = [6,5,4,3,2,1,0]
    return dados

Valores = [0,1,2,3,4,5,6]

print(Valores)
Valores = ModificaValores(Valores)
print(Valores)

^{See working on Ideone | Repl.it}

And why doesn’t it work when you assign the new list?

For you, when you define dados being a new list, defines a new object in memory, with a new address, which will exist only in the scope of the function. When you assign dados for Valores, you break the reference that Valores had with the external object and passes to reference the object dados local, but this change is made in the local scope, not being reflected to the external scope - in other words, you have an object Valores in the global scope and other in the local scope, being different from each other. The modifications you make in Valores, in the local scope, will not be reflected in Valores of the global scope, as they are different objects.