Published: 日 02 九月 2018

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。

1

2

3

4

5

6

7

8>>> # 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:

1

2

3

4

5

6>>> 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：

1

2>>> 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"):

1

2

3

4

5

6

7

8>>> 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():

1

2

3

4

5

6

7

8

9>>> 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:

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17>>> 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：

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17>>> 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:

1

2

3

4

5

6

7

8

9>>> 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:

1

2

3

4

5

6>>> 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:

1

2

3

4

5

6>>> 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:

1

2

3

4

5

6

7

8>>> 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:

1

2

3

4

5

6

7

8>>> 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:

1

2

3

4

5

6

7

8>>> 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：

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23# -*- 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()

条件循环while

代码：

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23# -*- 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()

参考资料

python测试等IT技术支持qq群： 144081101(后期会录制视频存在该群群文件) 591302926 567351477 钉钉免费群：21745728

道家技术-手相手诊看相中医等钉钉群21734177 qq群：391441566 184175668 338228106 看手相、面相、舌相、抽签、体质识别。服务费50元每人次起。请联系钉钉或者微信pythontesting