良好的编码风格:使用临时变量列表长度与否？

这个问题与python有关,但它实际上对很多语言都很常见.看看那段代码:

import sys
people = ['John', 'Jack', 'Charles']
cities = ['London', 'Liverpool', 'Manchester']
if (len(cities) != len(people)):
    print "Error! Length of cities list \
    (%s) differs from length of people list (%s)" % (len(cities), len(people))
    sys.exit(1)
for i in xrange (len(cities)):
    print "Hello, %s from %s" %(people[i], cities[i])

似乎是对的.但是在这个小片段len被称为3到5次(如果列表长度相等则为3次,如果列表长度不同则为5次).我们需要重写这样的代码吗？

import sys
people = ['John', 'Jack', 'Charles']
cities = ['London', 'Liverpool', 'Manchester']
people_count = len(people)
cities_count = len(cities)
if (cities_count != people_count):
    print "Error! Length of cities list \
    (%s) differs from length of people list (%s)" % (cities_count, people_count)
    sys.exit(1)
for i in xrange (cities_count):
    print "Hello, %s from %s" % (people[i], cities[i])

换句话说:在哪些情况下我们需要临时变量来存储列表长度,在哪些情况下不需要它？