Why does this happen?
x = [[]] * 4
x[0].append('x') -> [['x'], ['x'], ['x'], ['x']]
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1Because you are copying the same list four times. And since in your list you have 4 lists all that point to the same memory space, if you modify one of them, the change will affect all.rubik– rubik2012-03-04 11:44:04 +00:00Commented Mar 4, 2012 at 11:44
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Also, see this entry in the Python FAQBioGeek– BioGeek2012-03-04 16:07:19 +00:00Commented Mar 4, 2012 at 16:07
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1 Answer
the same instance of [] is being duplicated, so when you append to the first one 'x', you actually append it to all - because they are all the same object!
The right way to do it is to explicitly create a new list instance each time:
x = [[] for _ in range(4)]
3 Comments
Niklas B.
I added a possible solution to this answer. Hope you don't mind :)
alexis
To be clear: The instance [] is not being duplicated. The expression
[[]] * 4 creates a list with four references to the same instance of [].
Frg
[[]] * 4 looked like a nice trick to save some typing, but it bites people in the back afterwards by creating only two real lists and 3 references, which isn't so obvious. I used to do that too, but from now on I'll only use the list comprehension suggested here. Well, for such a small list [[],[],[],[]] is also fine. No need to show off using [[]] * 4. :)