本文教程目录

5循环

语法基础

for语句

Python的for语句针对序列(列表或字符串等)中的子项进行循环，按它们在序列中的顺序来进行迭代。

>>> # Measure some strings: ... words = ['cat', 'window', 'defenestrate'] >>> for w in words: ... print(w, len(w)) ... cat 3 window 6 defenestrate 12

在迭代过程中修改迭代序列不安全，可能导致部分元素重复两次，建议先拷贝:

>>> for w in words[:]: # Loop over a slice copy of the entire list. ... if len(w) > 6: ... words.insert(0, w) ... >>> words ['defenestrate', 'cat', 'window', 'defenestrate']

range()函数

内置函数 range()生成等差数值序列：

>>> range(10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

range(10) 生成了一个包含10个值的链表，但是不包含最右边的值。默认从0开始，也可以让range 从其他值开始，或者指定不同的增量值(甚至是负数，有时也称"步长"）:

>>> range(5, 10) [5, 6, 7, 8, 9] >>> range(0, 10, 3) [0, 3, 6, 9] >>> range(-10, -100, -30) [-10, -40, -70] >>> range(-10, -100, 30) []

如果迭代时需要索引和值可结合使用range()和len():

>>> a = ['Mary', 'had', 'a', 'little', 'lamb'] >>> for i in range(len(a)): ... print(i, a[i]) ... 0 Mary 1 had 2 a 3 little 4 lamb

不过使用enumerate()更方便，参见后面的介绍。

break和continue语句及循环中的else子句

break语句和C中的类似，用于终止当前的for或while循环。

循环可能有else 子句；它在循环迭代完整个列表(对于 for)后或执行条件为false(对于 while)时执行，但循环break时不会执行。这点和try...else而不是if...else相近。请看查找素数的程序:

>>> for n in range(2, 10): ... for x in range(2, n): ... if n % x == 0: ... print(n, 'equals', x, '*', n//x) ... break ... else: ... # loop fell through without finding a factor ... print(n, 'is a prime number') ... 2 is a prime number 3 is a prime number 4 equals 2 * 2 5 is a prime number 6 equals 2 * 3 7 is a prime number 8 equals 2 * 4 9 equals 3 * 3

continue语句也是从C而来，它表示退出当次循环，继续执行下次迭代。通常可以用if...else替代，请看查找偶数的实例：

>>> for n in range(2, 10): ... for x in range(2, n): ... if n % x == 0: ... print(n, 'equals', x, '*', n//x) ... break ... else: ... # loop fell through without finding a factor ... print(n, 'is a prime number') ... 2 is a prime number 3 is a prime number 4 equals 2 * 2 5 is a prime number 6 equals 2 * 3 7 is a prime number 8 equals 2 * 4 9 equals 3 * 3

pass

pass语句什么也不做。它语法上需要，但是实际什么也不做场合，也常用语以后预留以后扩展。例如:

>>> while True: ... pass # Busy-wait for keyboard interrupt (Ctrl+C) ... >>> class MyEmptyClass: ... pass ... >>> def initlog(*args): ... pass # Remember to implement this! ...

循环技巧

在字典中循环时，关键字和对应的值可以使用 items() 方法同时获取:

>>> knights = {'gallahad': 'the pure', 'robin': 'the brave'} >>> for k, v in knights.items(): ... print(k, v) ... gallahad the pure robin the brave

在序列中循环时 enumerate() 函数同时得到索引位置和对应值:

>>> for i, v in enumerate(['tic', 'tac', 'toe']): ... print(i, v) ... 0 tic 1 tac 2 toe

同时循环两个或更多的序列，可以使用 zip() 打包:

>>> questions = ['name', 'quest', 'favorite color'] >>> answers = ['lancelot', 'the holy grail', 'blue'] >>> for q, a in zip(questions, answers): ... print('What is your {0}? It is {1}.'.format(q, a)) ... What is your name? It is lancelot. What is your quest? It is the holy grail. What is your favorite color? It is blue.

需要逆向循环序列的话，调用 reversed() 函数即可:

>>> for i in reversed(xrange(1, 10, 2)): ... print(i) ... 9 7 5 3 1

使用 sorted() 函数可排序序列，它不改动原序列，而是生成新的已排序的序列:

>>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana'] >>> for f in sorted(set(basket)): ... print f ... apple banana orange pear

若要在循环时修改迭代的序列，建议先复制。

>>> import math >>> raw_data = [56.2, float('NaN'), 51.7, 55.3, 52.5, float('NaN'), 47.8] >>> filtered_data = [] >>> for value in raw_data: ... if not math.isnan(value): ... filtered_data.append(value) ... >>> filtered_data [56.2, 51.7, 55.3, 52.5, 47.8]

循环

代码：

# -*- coding: utf-8 -*- # Author: xurongzhong#126.com wechat:pythontesting qq:37391319 # 技术支持 钉钉群：21745728(可以加钉钉pythontesting邀请加入) # qq群：144081101 591302926 567351477 # CreateDate: 2018-6-12 # bowtie.py # Draw a bowtie from turtle import * def polygon(n, length): """Draw n-sided polygon with given side length.""" for _ in range(n): forward(length) left(360/n) def main(): """Draw polygons with 3-9 sides.""" for n in range(3, 10): polygon(n, 80) exitonclick() main()

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条件循环while

代码：

# -*- coding: utf-8 -*- # Author: xurongzhong#126.com wechat:pythontesting qq:37391319 # 技术支持 钉钉群：21745728(可以加钉钉pythontesting邀请加入) # qq群：144081101 591302926 567351477 # CreateDate: 2018-6-12 # spiral.py # Draw spiral shapes from turtle import * def spiral(firststep, angle, gap): """Move turtle on a spiral path.""" step = firststep while step > 0: forward(step) left(angle) step -= gap def main(): spiral(100, 71, 2) exitonclick() main()