I was trying to create a list with n empty lists inside of it, but faced an interesting situation.
Firstly I tried the following:
>>> n = 3>>> list([]) * n[]It creates an empty list.
After that I tried the following line, which creates an empty list, too:
>>> list(list()) * n[]But when I try the same with literals,
>>> [[]] * n [[], [], []]it gives the correct output. Can someone explain why?
Best Answer
list(...) is not interchangeable with [...]. You can wrap square brackets around things to get nested lists, but wrapping things in list() doesn't have the same effect.
When you write
[[]]you get a list with a single element. That element is[].When you write
list([])thelistconstructor takes an iterable and creates a list with the same elements. If you wrotelist(['foo', 'bar'])the result would be['foo', 'bar']. Writinglist([])yields[].This form is more useful when you pass iterables that aren't lists. Other examples:
list('abc') == ['a', 'b', 'c'] # strings are iterables over their characterslist(('foo', 'bar')) == ['foo', 'bar'] # tuplelist({'foo', 'bar'}) == ['foo', 'bar'] # set; could get ['bar', 'foo'] insteadlist(list())is equivalent tolist([]). And we've seen that's in turn equivalent to[].
Stay away from [[]] * n when creating a multidimensional list. It will create a list with n references to the same list object and will bite you when you try to modify the internal lists. It's a common mistake that everyone makes. Use this instead, which will create a separate empty list for each slot:
[[] for i in range(n)]Further reading:
- How to create a multi-dimensional list
- List of lists changes reflected across sublists unexpectedly