How variables are passed in a Python function?

Question

If I run the following code in Python 2.7, I get [2., 2., 2.] printed for both a and b. Why does b change together with a? Many thanks!

def test_f(x):
    a = np.zeros(3)
    b = a
    for i in range(3):
        a[i] += x
    print a
    print b
    return 0

test_f(2)

because a and b refer to the same memory location holding the list — danidee, Feb 05 '16 at 01:19

javad · Answer 1 · 2016-02-05T01:26:35.520

5

Because b and a are referring to the same list in memory. b = a does not create a new copy of a. Try this and see the difference:

def test_f(x):
    a = np.zeros(3)
    b = a.copy()
    for i in range(3):
        a[i] += x
    print a
    print b
    return 0

test_f(2)

b = a.copy() will create a new copy that exactly resembles the elements of a, whereas b=a just creates a new reference to the exisiting list.

edited Feb 05 '16 at 01:26

answered Feb 05 '16 at 01:16

javad

498
4
15

1

I think for readability ``b=a.copy()`` or ``b=np.array(a, copy=True)`` would be better since you explicitly mention that a copy is made. – MSeifert Feb 05 '16 at 01:19
2

Also, you need only one colon between the brackets. – zondo Feb 05 '16 at 01:21
1

I agree with @MSeifert; moreover the `[::]` only needs one colon – cat Feb 05 '16 at 01:21
Thanks. You are right. I fixed it! – javad Feb 05 '16 at 01:22
I agree with MSeifert, but this is an easier way to code anyways... This point is kinda subtle so I guess it will be helpful to mark the "copy" explicitly. Thanks – nusjjsun Feb 05 '16 at 01:24

score 2 · Answer 2 · answered Feb 05 '16 at 01:20

2

numpy will use a pointer to copy unless you tell it otherwise:

import numpy as np

def test_f(x):
    a = np.zeros(3)
    b = np.copy(a)
    for i in range(3):
        a[i] += x
    print a
    print b
    return 0

test_f(2)

[ 2.  2.  2.]
[ 0.  0.  0.]

answered Feb 05 '16 at 01:20

Keith Brodie

657
3
17

Thanks, Keith! This is a very clear explanation! – nusjjsun Feb 05 '16 at 01:24

How variables are passed in a Python function?

2 Answers2