Python list() vs list comprehension building speed

Question

This is interesting; list() to force an iterator to get the actual list is so much faster than [x for x in someList] (comprehension).

Is this for real or is my test just too simple? Below is the code:

import time    

timer = time.clock()
for i in xrange(90):

    #localList = [x for x in xrange(1000000)]   #Very slow, took me 6.8s
    localList = list(xrange(1000000))           #Very fast, took me 0.9s

    print localList[999999] #make sure list is really evaluated.

print "Total time: ", time.clock() - timer

Answer 1

The list comprehension executes the loop in Python bytecode, just like a regular for loop.

The list() call iterates entirely in C code, which is far faster.

The bytecode for the list comprehension looks like this:

>>> import dis
>>> dis.dis(compile("[x for x in xrange(1000000)]", '<stdin>', 'exec'))
  1           0 BUILD_LIST               0
              3 LOAD_NAME                0 (xrange)
              6 LOAD_CONST               0 (1000000)
              9 CALL_FUNCTION            1
             12 GET_ITER            
        >>   13 FOR_ITER                12 (to 28)
             16 STORE_NAME               1 (x)
             19 LOAD_NAME                1 (x)
             22 LIST_APPEND              2
             25 JUMP_ABSOLUTE           13
        >>   28 POP_TOP             
             29 LOAD_CONST               1 (None)
             32 RETURN_VALUE        

The >> pointers roughly give you the boundaries of the loop being executed, so you have 1 million STORE_NAME, LOAD_NAME and LIST_APPEND steps to execute in the Python bytecode evaluation loop.

list() on the other hand just grabs the values from the xrange() iterable directly by using the C API for object iteration, and it can use the length of the xrange() object to pre-allocate the list object rather than grow it dynamically.