Python list comprehension expensive

Question

Im trying to find the effeciency of list comprehension but it look like its more expensive than a normal function operation. Can someone explain?

def squares(values):
    lst = []
    for x in range(values):
        lst.append(x*x)
    return lst

def main():
    t = timeit.Timer(stmt="lst = [x*x for x in range(10)]")
    print t.timeit()
    t = timeit.Timer(stmt="squares",setup="from __main__ import squares")
    print t.timeit()

    lst = [x*x for x in range(10)]
    print lst
    print squares(10)



----Output:---
2.4147507644
0.0284455255965
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

For the same output, the normal function calculates in very less time compared to the list comprehension.

I thought the list comprehension is more effecient.

Answer 1

You are never calling your squares function, so it is not doing anything.

List comprehensions are in fact faster:

>>> import timeit
>>> def squares(values):
...     lst = []
...     for x in range(values):
...         lst.append(x*x)
...     return lst
... 
>>> def squares_comp(values):
...     return [x*x for x in range(values)]
... 
>>> timeit.timeit('f(10)', 'from __main__ import squares as f')
3.9415171146392822
>>> timeit.timeit('f(10)', 'from __main__ import squares_comp as f')
2.3243820667266846

If you use the dis module to look at the bytecode for each function, you can see why:

>>> import dis
>>> dis.dis(squares)
  2           0 BUILD_LIST               0
              3 STORE_FAST               1 (lst)

  3           6 SETUP_LOOP              37 (to 46)
              9 LOAD_GLOBAL              0 (range)
             12 LOAD_FAST                0 (values)
             15 CALL_FUNCTION            1
             18 GET_ITER            
        >>   19 FOR_ITER                23 (to 45)
             22 STORE_FAST               2 (x)

  4          25 LOAD_FAST                1 (lst)
             28 LOAD_ATTR                1 (append)
             31 LOAD_FAST                2 (x)
             34 LOAD_FAST                2 (x)
             37 BINARY_MULTIPLY     
             38 CALL_FUNCTION            1
             41 POP_TOP             
             42 JUMP_ABSOLUTE           19
        >>   45 POP_BLOCK           

  5     >>   46 LOAD_FAST                1 (lst)
             49 RETURN_VALUE        
>>> dis.dis(squares_comp)
  2           0 BUILD_LIST               0
              3 LOAD_GLOBAL              0 (range)
              6 LOAD_FAST                0 (values)
              9 CALL_FUNCTION            1
             12 GET_ITER            
        >>   13 FOR_ITER                16 (to 32)
             16 STORE_FAST               1 (x)
             19 LOAD_FAST                1 (x)
             22 LOAD_FAST                1 (x)
             25 BINARY_MULTIPLY     
             26 LIST_APPEND              2
             29 JUMP_ABSOLUTE           13
        >>   32 RETURN_VALUE        

The squares function looks up the .append() method of the list in each iteration, and calls it. The .append() function has to grow the list by one element each time it is called.

The list comprehension on the other hand doesn't have to do that work. Instead, python uses the LIST_APPEND bytecode, which uses the C API to append a new element to the list, without having to do the lookup and a python call to the function.