Function that modifies an argument outside that function without using "return" python

Question

I use python 2.7. Say I have an array X and a function that centers this array according to means :

import numpy as np
X = np.array([[float(i+j) for i in range(3)] for j in range(3)])

If we print X at this point

X = array([[ 0.,  1.,  2.],
           [ 1.,  2.,  3.],
           [ 2.,  3.,  4.]])

Now let's compute the mean (according to each column) and write the function that centers each X[i,j] according to the j-th column's mean.

means = X.means(axis=0) #mean for each column

We print means = array([ 1., 2., 3.]) (seem's legit).

def center(arr, means) :
    for i, mean in enumerate(means) :
        arr[:, i] -= mean
    # Instructions
    # end of the function without return X or something

Let' call this function on X : center(X), and print again X, one will have :

X = array([[-1., -1., -1.],
           [ 0.,  0.,  0.],
           [ 1.,  1.,  1.]])

So, X is modified whereas it shouldn't be since it is not returned.

My question is twofold : 1-How come X is modified outside the function whereas I don't return it 2- Is there something to do to avoid that modification that can lead to big confusion

The solution I found is to np.copy the arr before the for loop, (inside the center function), and deal with the copy to keep X as it is.

Note that this problem doesn't arise when dealing with int or float.

PS : I suppose that the problem rely on how the arrays are stored and the way broadcasting is made in Python but I believe there is a way to get a better grip on it

Thanks for your time

The link of @fuglede should answer your question. But normally you don't need the for loop, just `X -= X.mean(0)` should do the job. — phi, Jun 04 '18 at 18:48
When you pass an object as a function argument - you are passing a *reference* to that object. If the object is mutable, and you change part of it, that change will be seen in the original - as you found out. To avoid that you need to make a copy. Reading: [Facts and myths about Python names and values](https://nedbatchelder.com/text/names.html), [9.1 and 9.2 of the Tutorial](https://docs.python.org/3/tutorial/classes.html#a-word-about-names-and-objects), [naming and Binding](https://docs.python.org/3/reference/executionmodel.html#naming-and-binding). — wwii, Jun 04 '18 at 19:09

Function that modifies an argument outside that function without using "return" python

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