Python For 循环常用示例

格式

for {variable} in {some-sequence-type}:
    {python-statements}
else:
    {python-statements}

常见序列类型：lists，strings，tuples等

示例

Numbers循环

# list
>>> for i in [1, 2, 3, 4, 5]:   
...     print(i)
...
1
2
3
4
5

# 可以把list替换成tuple
>>> for i in (1, 2, 3, 4, 5):
...     print(i)
...
1
2
3
4
5

Strings循环

>>> names = ['john', 'raj', 'lisa']
>>> for i in names:
...     print(i)
...
john
raj
lisa

Range默认

>>> for i in range(5):
...     print(i)
...
0
1
2
3
4

Range(自定义起始和结束值)

>>> for i in range(1, 6):
...     print(i)
...
1
2
3
4
5

Range(指定步长)

>>> for i in range(1, 6, 2):    # 未指定，默认为1
...     print(i)
...
1
3
5

Range(负值)

>>> for i in range(4, -5, -2):
...     print(i)
...
4
2
0
-2
-4

Continue

>>> names = ['john', 'lisa', 'raj', 'lisa']
>>> for i in names:
...     if i != 'lisa':
...             continue    # i不等于lisa则跳过本次循环
...     print(i)
...
lisa
lisa

Break

>>> names = ['john', 'lisa', 'raj', 'lisa']
>>> for i in names:
...     if i == 'raj':
...             break    # i等于raj则退出整个for循环。
...     print(i)
...
john
lisa

有些场景可以使用return替换break，效果是一样的，见《Python Tricks: The BookA Buffet of Awesome Python Features》

For + Else

>>> names = ['john', 'lisa', 'raj', 'lisa']
>>> for i in names:
...     print(i)
... else:
...     print('--Done--')
...
john
lisa
raj
lisa
--Done--

for循环结束后，执行else语句

For + Break + Else

>>> names = ['john', 'lisa', 'raj', 'lisa']
>>> for i in names:
...     if i == 'raj':
...             break
...     print(i)
... else:               # 使用break将不会执行到else语句
...     print('--Done--')
...
john
lisa

多重循环

>>> distros = ["centos", "redhat", "ubuntu"]
>>> arch = ["32-bit", "64-bit"]
>>> for i in distros:
...     for j in arch:
...             print('{} {}'.format(i, j))
...     print('--------')
...
centos 32-bit
centos 64-bit
--------
redhat 32-bit
redhat 64-bit
--------
ubuntu 32-bit
ubuntu 64-bit
--------

Lists中的list

>>> multiple_state_lists = [ ["CA","NV","UT"], ["NJ","NY","DE"]]
>>> for state_list in multiple_state_lists:
...     for state in state_list:
...             print(state)
...
CA
NV
UT
NJ
NY
DE

Enumerate(输出索引)

>>> names = ['john', 'lisa', 'raj', 'lisa']
>>> for index, item in enumerate(names):
...     print('{} {}'.format(index, item))
...
0 john
1 lisa
2 raj
3 lisa

Zip(多list使用)

>>> names = ['john', 'lisa', 'raj', 'lisa']
>>> ages = [28, 25, 31, 20]
>>> for item in zip(names, ages):
...     print('{name}:{age}'.format(name=item[0], age=item[1]))
...
john:28
lisa:25
raj:31
lisa:20

Dict(items)

>>> peoples = {'john': 28, 'lisa': 25, 'raj': 31}
>>> for k, v in peoples.items():
...     print('{name}:{age}'.format(name=k, age=v))
...
raj:31
john:28
lisa:25

推导式(list)

>>> ['Hi' for _ in range(5)]
['Hi', 'Hi', 'Hi', 'Hi', 'Hi']

推导式(dict)

>>> {i: i * 2 for i in range(5)}
{0: 0, 1: 2, 2: 4, 3: 6, 4: 8}

推导式(生成器)

>>> [i for i in (i for i in range(5))]
[0, 1, 2, 3, 4]

推导式(if语句)

>>> [i for i in range(5) if i % 2]
[1, 3]