Confused about how the assignment operator works

Question

There is something about the way python assigns a value to a variable that I'm not fully comprehending. Why is it that this:

charlie = ['d', 'o', 'p', 'e']
beth = charlie
beth[0] = charlie[1]
print charlie

gives me this:

['o', 'o', 'p', 'e']

As far as I understand the assignment operator only works one way, thus this code should only change the first index of beth, not charlie. So what gives?

This should help clear up the confusion: http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html#other-languages-have-variables — brianpck, Nov 10 '16 at 21:16
Also, read [this section](http://www.nedbatchelder.com/text/names.html#h_assignment) of Ned Batchelder's Facts and Myths about Python Names and Values. You might as well read the whole thing, really. — juanpa.arrivillaga, Nov 10 '16 at 21:21
`beth = charlie` does not create a copy, just 2 names for the same thing. — takendarkk, Nov 10 '16 at 21:23

rassar · Accepted Answer · 2016-11-10T21:16:17.390

1

When you say beth = charlie, beth is now an alias of charlie. That means that anything that happens to beth now happens to charlie.

http://gestaltrevision.be/wiki/python/aliases

In order for this not to happen, you can try beth = list(charlie) or beth = charlie[:].

edited Nov 10 '16 at 21:16

answered Nov 10 '16 at 21:14

rassar

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if you want for beth to be a new list, you can do beth = list(charlie), these will now be different objects. – chatton Nov 10 '16 at 21:15
1

or `beth = charlie[:]` – rassar Nov 10 '16 at 21:15

score 0 · Answer 2 · edited Nov 10 '16 at 21:18

0

You just assigned the same list to two variables, so charlie and beth are the same object.

edited Nov 10 '16 at 21:18

John Kugelman

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answered Nov 10 '16 at 21:14

jonzee

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Confused about how the assignment operator works

2 Answers2