Python list simultaneous update

Question

This seems to be a gotcha for me, I couldnt figure this out

>>> from collections import Counter
>>> tree = [Counter()]*3
>>> tree
[Counter(), Counter(), Counter()]
>>> tree[0][1]+=1
>>> tree
[Counter({1: 1}), Counter({1: 1}), Counter({1: 1})]

Why does updating one Counter updates everything?

Because of this line `[Counter()]*3`. You're not creating 3 unique Counters. You're creating one list with three references to the same Counter object. — Henrik Andersson, Jul 29 '13 at 18:59
You've got a list with three references to the same counter. Try a list comprehension instead. — Waleed Khan, Jul 29 '13 at 19:00

score 5 · Accepted Answer · answered Jul 29 '13 at 19:02

Using [x] * 3, the list references same item(x) three times.

>>> from collections import Counter
>>> tree = [Counter()] * 3
>>> tree[0] is tree[1]
True
>>> tree[0] is tree[2]
True
>>> another_counter = Counter()
>>> tree[0] is another_counter
False

>>> for counter in tree: print id(counter)
...
40383192
40383192
40383192

Use list comprehension as Waleed Khan commented.

>>> tree = [Counter() for _ in range(3)]
>>> tree[0] is tree[1]
False
>>> tree[0] is tree[2]
False

>>> for counter in tree: print id(counter)
...
40383800
40384104
40384408

Also, `for counter in tree: print id(counter)` should give a better understanding. — Henrik Andersson, Jul 29 '13 at 19:03

arshajii · Answer 2 · 2013-07-29T19:13:08.367

1

[Counter()]*3 produces a list containing the same Counter instance 3 times. You can use

[Counter() for _ in xrange(3)]

to create a list of 3 independent Counters.

>>> from collections import Counter
>>> tree = [Counter() for _ in xrange(3)]
>>> tree[0][1] += 1
>>> tree
[Counter({1: 1}), Counter(), Counter()]

In general, you should be cautious when multiplying lists whose elements are mutable.

edited Jul 29 '13 at 19:13

answered Jul 29 '13 at 19:02

arshajii

127,459
24
238
287

score 1 · Answer 3 · answered Jul 29 '13 at 19:02

tree = [Counter()]*3 creates one counter and three references to it; you could write it as:

c = Counter()
tree = [c, c, c]

You want three counters:

>>> from collections import Counter
>>> tree = [Counter() for _ in range(3)]
>>> tree[0][1]+=1
>>> tree
[Counter({1: 1}), Counter(), Counter()]
>>>

Python list simultaneous update

3 Answers3