Copied variable changes the original?

I have a simple problem in Python that is very very strange.

def estExt(matriz,erro):
    # (1) Determinar o vector X das soluções
    print ("Matrix after:");
    print(matriz);

    aux=matriz;
    x=solucoes(aux); # IF aux is a copy of matrix, why the matrix is changed??

    print ("Matrix before: ");
    print(matriz)

...

As you see below, the matrix matriz is changed in spite of the fact that aux is the one being changed by the function solucoes().

Matrix before:
[[7, 8, 9, 24], [8, 9, 10, 27], [9, 10, 8, 27]]

Matrix after:
[[7, 8, 9, 24], [0.0, -0.14285714285714235, -0.2857142857142847, -0.42857142857142705],
[0.0, 0.0, -3.0, -3.0000000000000018]]

Answers:

Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.

Method 1

The line

aux=matriz;

Does not make a copy of matriz, it merely creates a new reference to matriz named aux. You probably want

aux=matriz[:]

Which will make a copy, assuming matriz is a simple data structure. If it is more complex, you should probably use copy.deepcopy

aux = copy.deepcopy(matriz)

As an aside, you don’t need semi-colons after each statement, python doesn’t use them as EOL markers.

Method 2

Use copy module

aux = copy.deepcopy(matriz) # there is copy.copy too for shallow copying

Minor one: semicolons are not needed.

Method 3

aux is not a copy of matrix, it’s just a different name that refers to the same object.

Use the copy module to create actual copies of your objects.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0