GA version "Hello World!" in Python

helloworld.py

#!/usr/bin/env python
# -*- encoding: utf-8 -*-

from itertools import izip
from random import randint
from random import random as rand

GA_POPSIZE = 2048
GA_MAXITER = 16384
GA_ELITERATE = 0.1
GA_ELITESIZE = int(GA_POPSIZE * GA_ELITERATE)
GA_MUTATIONRATE = 0.25
GA_TARGET = 'hello world!'
GA_TARGETSIZE = len(GA_TARGET)

class Candidate:
    def __init__(self, str_):
        self.str = str_

    @property
    def fitness(self):
        fitness = 0
        for c1, c2 in izip(self.str, GA_TARGET):
            fitness += abs(ord(c1) - ord(c2))
        return fitness

def randchar():
    return chr(randint(32, 121))

def init_population():
    population = []

    for _ in xrange(GA_POPSIZE):
        str_ = ''.join((randchar() for _ in xrange(GA_TARGETSIZE)))
        member = Candidate(str_)
        population.append(member)

    return population

def mutate_char(char):
    delta = randint(1, 90)
    return chr((ord(char) - 32 + delta) % 90 + 32)

def mutate(member):
    pos = randint(0, GA_TARGETSIZE - 1)
    newchr = mutate_char(member.str[pos])
    member.str = member.str[:pos] + newchr + member.str[pos+1:]
    return member

def mate(member1, member2):
    spos = randint(1, GA_TARGETSIZE - 1)
    newstr = member1.str[:spos] + member2.str[spos:]
    member = Candidate(newstr)

    if rand() < GA_MUTATIONRATE:
        member = mutate(member)

    return member

def new_generation(population):
    population.sort(key=lambda c: c.fitness)

    for _ in xrange(GA_POPSIZE - GA_ELITESIZE):
        i1 = randint(0, GA_POPSIZE - 1)
        i2 = randint(0, GA_POPSIZE - 1)

        member = mate(population[i1], population[i2])
        population.append(member)

    population = population[:GA_ELITESIZE] + population[GA_POPSIZE:]
    return population

def main():
    population = init_population()

    for _ in xrange(GA_MAXITER):
        population = new_generation(population)
        print population[0].str

        if not population[0].fitness: break

if __name__ == '__main__':
    main()