How to create a list or tuple of empty lists in Python?

Question

I need to incrementally fill a list or a tuple of lists. Something that looks like this:

result = [] firstTime = True for i in range(x):     for j in someListOfElements:         if firstTime:             result.append([f(j)])         else:             result[i].append(j) 

In order to make it less verbose an more elegant, I thought I will preallocate a list of empty lists

result = createListOfEmptyLists(x) for i in range(x):     for j in someListOfElements:         result[i].append(j) 

The preallocation part isn't obvious to me. When I do result = [[]] * x, I receive a list of x references to the same list, so that the output of the following

result[0].append(10) print result 

is:

[[10], [10], [10], [10], [10], [10], [10], [10], [10], [10]] 

I can use a loop (result = [[] for i in range(x)]), but I wonder whether a "loopless" solution exists.

Is the only way to get what I'm looking for

Answer 1

result = [list(someListOfElements) for _ in xrange(x)] 

This will make x distinct lists, each with a copy of someListOfElements list (each item in that list is by reference, but the list its in is a copy).

If it makes more sense, consider using copy.deepcopy(someListOfElements)

Generators and list comprehensions and things are considered quite pythonic.