infinite assignment in python list?

Question

I've come cross this question. Code:

>>> values = [0, 1, 2]
>>> values[1] = values
>>> values
[0, [...], 2]

The result I expect is:

[0, [0, 1, 2], 2]

Is this an infinite assignment for python list? What is behind the scene?

Thanks.

`What is behind the scene`: read this [Q&A:`What is […] in Python`](http://stackoverflow.com/questions/17160162/what-is-in-python-2-7/17163521#17163521) — Grijesh Chauhan, Jul 20 '13 at 14:12

score 6 · Accepted Answer · answered Jul 20 '13 at 14:07

6

You have a recursive list there. values[1] is a reference to values. If you want to store the value of values you need to copy it, the easiest way to do so is

values[1] = values[:]

answered Jul 20 '13 at 14:07

filmor

30,840
6
50
48

score 3 · Answer 2 · answered Jul 20 '13 at 14:07

3

You put the same list as the second element inside itself. So the second element of the internal list is itself again.

You need to copy initial list to avoid recursion:

>>> values[1] = values[:]

answered Jul 20 '13 at 14:07

ovgolovin

13,063
6
47
78

score 2 · Answer 3 · answered Jul 20 '13 at 14:14

2

You need to copy it:

values[1] = values[:]

answered Jul 20 '13 at 14:14

Michael

15,386
36
94
143

**Not** Copy, (assign) – Grijesh Chauhan Jul 20 '13 at 14:15
2

This code is indeed a (shallow) copy, since slicing replicates the list's structure (it does not copy the members though!). – filmor Jul 20 '13 at 14:17
@GrijeshChauhan copy of references maybe? – ersran9 Jul 20 '13 at 14:17
1

@ersran9 Yes, filmor is more correct its called shadow copy actually (not deep copy) – Grijesh Chauhan Jul 20 '13 at 14:20
@GrijeshChauhan Yes, that's what I meant by copy of references. Sorry for the confusion. – ersran9 Jul 20 '13 at 14:21
@filmor and GrijeshChauhan Would you briefly explain the concept (shallow and deep) copy to me, take my code as an example? – lulyon Jul 20 '13 at 14:25
1

http://stackoverflow.com/questions/323689/python-list-slice-syntax-used-for-no-obvious-reason – filmor Jul 20 '13 at 14:27
1

@lulyon Look at image here: [Shallow vs. Deep Cloning](http://www.csharp411.com/c-object-clone-wars/) – Grijesh Chauhan Jul 20 '13 at 14:28
1

@lulyon better: [diagram-1](http://www.roguewave.com/portals/0/products/legacy-hpp/docs/tlsug/10.2.html) [diagram-2](http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/Collections/Articles/Copying.html) – Grijesh Chauhan Jul 20 '13 at 14:30
1

@GrijeshChauhan Very instructive diagrams. Deeply greatful! – lulyon Jul 20 '13 at 14:34

score 1 · Answer 4 · answered Jul 20 '13 at 14:50

>>> values = [0, 1, 2]
>>> values[1] = values

you are saying that values[1] is values, which is [0, 1, 2], with values instead of 1, and now values is [0, 1, 2], with values instead of 1, [0, 1, 2], with values instead of 1, [0, 1, 2], with values instead of 1,[0, 1, 2], with values instead of 1, [0, 1, 2], with values instead of 1 ... ... ...

infinite assignment in python list?

4 Answers4