非常奇怪的Python变量作用域行为

Question

I'm having a problem with Python 2.7 that is driving me insane.

I'm passing an array to some functions and altough that variable is suposed to be local, in the end the value of the variable inside main is changed.

I'm a bit new to Python, but this goes against any common sense I got.

Any ideas of what I'm doing wrong?

def mutate(chromo):
    # chooses random genes and mutates them randomly to 0 or 1
    for gene in chromo:
        for codon in gene:
            for base in range(2):
                codon[randint(0, len(codon)-1)] = randint(0, 1)
    return chromo

def mate(chromo1, chromo2):
    return mutate([choice(pair) for pair in zip(chromo1, chromo2)])


if __name__ == '__main__':
    # top 3 is a multidimensional array with 3 levels (in here I put just 2 for simplicity)
    top3 = [[1, 0], [0, 0], [1, 1]]

    offspring = []
    for item in top3:
        offspring.append(mate(top3[0], item))

    # after this, top3 is diferent from before the for cycle

UPDATE Because Python passes by reference, I must make a real copy fo the arrays before using them, so the mate functions must be changed to:

import copy
def mate(chromo1, chromo2):
    return mutate([choice(pair) for pair in zip(copy.deepcopy(chromo1), copy.deepcopy(chromo2))])

Answer 1

The problem you are having is stemming from the fact that arrays and dictionaries in python are passed by reference. This means that instead of a fresh copy being created by the def and used locally you are getting a pointer to your array in memory...

x = [1,2,3,4]

def mystery(someArray):
     someArray.append(4)
     print someArray

mystery(x)
[1, 2, 3, 4, 4]

print x
[1, 2, 3, 4, 4]

Answer 2

You manipulate chromo , which you pass by reference. Therefore the changes are destructive... the return is therefore kind of moot as well ( codon is in gene and gene is in chromo ). You'll need to make a (deep) copy of your chromos , I think.

Answer 3

try changing

offspring.append(mate(top3[0], item)) to offspring.append(mate(top3[0][:], item[:]))

or use the list() function