Python: fastest way to create a list of n lists

Question

So I was wondering how to best create a list of blank lists:

[[],[],[]...] 

Because of how Python works with lists in memory, this doesn't work:

[[]]*n 

This does create [[],[],...] but each element is the same list:

d = [[]]*n d[0].append(1) #[[1],[1],...] 

Something like a list comprehension works:

d = [[] for x in xrange(0,n)] 

But this uses the Python VM for looping. Is there any way to use an implied loop (taking advantage of it being written in C)?

d = [] map(lambda n: d.append([]),xrange(0,10)) 

This is actually slower. :(

Answer 1

The probably only way which is marginally faster than

d = [[] for x in xrange(n)] 

is

from itertools import repeat d = [[] for i in repeat(None, n)] 

It does not have to create a new int object in every iteration and is about 15 % faster on my machine.

Edit: Using NumPy, you can avoid the Python loop using

d = numpy.empty((n, 0)).tolist() 

but this is actually 2.5 times slower than the list comprehension.