Python: strange list behaviour when initialising empty nested list

Question

Could someone explain why:

b = 10*[[]]
b[0].append(3)

b now is:

[[3], [3], [3], [3], [3], [3], [3], [3], [3], [3]]

but if I do:

b = [[] for _ in range(10)]
b[0].append(3)

b is now:

[[3], [], [], [], [], [], [], [], [], []]

Also note that

10*[[]] == [[] for [] in range(10)]

returns

True

(Tested in Python 3.6.1)

[List of lists changes reflected across sublists unexpectedly](https://stackoverflow.com/q/240178/2301450) — vaultah, Aug 23 '17 at 12:53

bakaDev · Accepted Answer · 2017-08-23T13:01:13.343

5

 because:
    >>> b = 10*[[]]
    >>> b[0] is b[1]
    True
    >>> b = [[] for _ in range(10)]
    >>> b[0] is b[1]
    False
    >>>

edited Aug 23 '17 at 13:01

answered Aug 23 '17 at 12:58

bakaDev

1 Answers1