Python: unpacking into array elements

Question

Why does the behavior of unpacking change when I try to make the destination an array element?

>>> def foobar(): return (1,2)
>>> a,b = foobar()
>>> (a,b)
 (1, 2)
>>> a = b = [0, 0] # Make a and b lists
>>> a[0], b[0] = foobar()
>>> (a, b)
 ([2, 0], [2, 0])

In the first case, I get the behavior I expect. In the second case, both assignments use the last value in the tuple that is returned (i.e. '2'). Why?

The behaviour of unpacking doesnt change, you are doing a multiple assignment in the 2nd example, not unpacking — jamylak, Apr 18 '13 at 03:13

score 3 · Accepted Answer · answered Apr 18 '13 at 02:56

3

When you do a = b = [0, 0], you're making both a and b point to the same list. Because they are mutable, if you change either, you change both. Use this instead:

a, b = [0, 0], [0, 0]

answered Apr 18 '13 at 02:56

Volatility

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Thank you, Volatility, for the suggestion. – Gordon Bean Apr 18 '13 at 03:00

score 2 · Answer 2 · answered Apr 18 '13 at 02:55

2

a = b = [0, 0] # Makes a and b the same list

answered Apr 18 '13 at 02:55

John La Rooy

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gnibbler, you beat me too it. :) I just realized my folly and returned to delete the question, and you had already answered. Thank you for the prompt reply. – Gordon Bean Apr 18 '13 at 02:59

Python: unpacking into array elements

2 Answers2