Within the Python docs essays section is a wonderful article by Guido van Rossum on optimizing code with Python. You should read the whole thing but I’m highlighting the bits I found most interesting and include a Python 3 version of the working code at the end.

The specific computer science problem he tries to solve is, How do you efficiently convert a list of integers that are ASCII values into a string.

It’s fascinating to see even Guido himself reason through the problem in normal fashion, starting with a simple solution and then trying out various optimizations.

His first take is simple and elegant:

def f1(list):
    string = ""
    for item in list:
        string = string + chr(item)
    return string

>>> int_list = [97, 98, 99]
>>> f1(int_list)
'abc'

And by version 7 he’s way, way faster. The progression and the way he tests the time execution of each is particularly interesting.

Here’s the complete Python 3 code (his example is older and uses Python 2):

import time
import functools
import array


def timing(f, n, a):
    print(f.__name__,)
    r = range(n)
    t1 = time.clock()
    for i in r:
        f(a)
        f(a)
        f(a)
        f(a)
        f(a)
        f(a)
        f(a)
        f(a)
        f(a)
        f(a)
    t2 = time.clock()
    print(round(t2 - t1, 3))


def f1(list):
    string = ""
    for item in list:
        string = string + chr(item)
    return string


def f2(list):
    return functools.reduce(lambda string, item: string + chr(item), list, "")


def f3(list):
    string = ""
    for character in map(chr, list):
        string = string + character
    return string


def f4(list):
    string = ''
    lchr = chr
    for item in list:
        string = string + lchr(item)
    return string


def f5(list):
    string = ''
    for i in range(0, 256, 16):  # 0, 16, 32...
        s = ''
        for character in map(chr, list[i:i + 16]):
            s = s + character
        string = string + s
    return string


def f6(list):
    return ''.join(map(chr, list))


def f7(list):
    return array.array('B', list).tostring()

testdata = list(range(256))
print(testdata)
testfuncs = f1, f2, f3, f4, f5, f6, f7
for f in testfuncs:
    print(f.__name__, f(testdata))
for f in testfuncs:
    timing(f, 100, testdata)