Question

Here is some code that I wrote using Python:

from math import sqrt
abundant_list = []

for i in range(12,28123+1):
    dividor_list = [1]
    for j in range(2, int(sqrt(i))+1):
        if i%j == 0:
            dividor_list.extend([i/j,j])
    if sum(dividor_list) > i:
        abundant_list.append(i)

print abundant_list

As you can see, the code is really trying to be efficient as much as possible.

There is any difference if I use list.append twice, or list.extend just once?
I know it can be minor differences, but I would really like to know that 🙂

Answer 1

import timeit

def append2x(foo):
    foo.append(1)
    foo.append(1)

def extend_lst(foo):
    foo.extend([1,1])

def extend_tup(foo):
    foo.extend((1,1))


l1 = []
l2 = []
l3 = []

print timeit.timeit('append2x(l1)',setup = 'from __main__ import append2x,l1')
print timeit.timeit('extend_lst(l2)',setup = 'from __main__ import extend_lst,l2')
print timeit.timeit('extend_tup(l3)',setup = 'from __main__ import extend_tup,l3')

Here’s a simple benchmark. My results (os-X, 10.5.8, core2duo, FWIW):

0.520906925201  #append
0.602569103241  #extend-list
0.357008934021  #extend-tuple

And the same ordering of the results my linux box (Ubuntu, x86-64 core i7):

0.307395935059  #append
0.319436073303  #extend-list
0.238317012787  #extend-tuple

To me, this says that extend is quicker than append, but that creating a list is relatively expensive compared to creating a tuple

---

EDIT

Pointed out in the comments below, because of the immutability of tuples, the interpreter can optimize the creation of the tuple out (it creates the tuple once and re-uses it over and over). If we change the code to:

def extend_lst(foo):  
    v = 1
    foo.extend([v,v]) 

def extend_tup(foo):
    v = 1
    foo.extend((v,v))

The timings are virtually identical:

0.297003984451  #append
0.344678163528  #extend-list
0.292304992676  #extend-tuple

Although tuple still consistently beats the list version and barely edges out the append version for all of the trials I have done.

One thing that I’m taking away from this is that if you’re iterating over an object that consists of all literals, choose a tuple over a list. If it doesn’t consist entirely of literals, then it really doesn’t matter whether you choose list or tuple.