【python】常用内置模块itertools的用法(迭代器)
文章目录
1. 简介
python
内置模块itertools
中封装了很多高效的迭代器,本文将讲述10个最为常用的迭代器函数。
2. 无限迭代器
2.1. count(start, [step])
输出自start
参数开始的无限长度的自然数序列,step
参数设置步长,可选。
>>> import itertools as its
>>> a = its.count(1, 2)
>>> for i in a:
... print(i)
...
1
3
5
...
使用快捷键Ctrl+C暂停打印
2.2. cycle( p)
将一个序列无限循环下去,p
参数指定待循环的序列。
>>> import itertools as its
>>> a = its.cycle('ABC')
>>> for i in a:
... print(i)
...
A
B
C
A
B
C
...
使用快捷键Ctrl+C暂停打印
2.3. repeat(elem, [n])
把一个元素重复n
次,如果不指定参数n
,则默认无限次。
>>> import itertools as its
>>> a = its.repeat(10, 3)
>>> for i in a:
... print(i)
...
10
10
10
3. 通用迭代器
3.1. accumulate(p, [func])
依次累加序列p
中的元素。
>>> import itertools as its
>>> a = its.accumulate([1, 2, 3, 4, 5])
>>> for i in a:
... print(i)
...
1 # 1
3 # 1+2
6 # 1+2+3
10 # 1+2+3+4
15 # 1+2+3+4+5
3.2. chain(p, q, […])
串联多个迭代器,形成一个更大的迭代器。
>>> import itertools as its
>>> a = its.chain('ABD', 'DEF')
>>> for i in a:
... print(i)
...
A
B
D
D
E
F
3.3. takewhile(pred, seq)
迭代选择器。
>>> import itertools as its
>>> a = its.takewhile(lambda x: x < 5, [1, 3, 5, 7])
>>> for i in a:
... print(i)
...
1
3
4. 组合迭代器
4.1. product(p, q, […], [repeat=1])
笛卡尔积,等效于嵌套的for
循环:
>>> import itertools as its
>>> a = its.product('ABC', repeat=2)
>>> for i in a:
... print(i)
...
('A', 'A')
('A', 'B')
('A', 'C')
('B', 'A')
('B', 'B')
('B', 'C')
('C', 'A')
('C', 'B')
('C', 'C')
等价于:
>>> for x in 'ABC':
... for y in 'ABC':
... print((x, y))
...
('A', 'A')
('A', 'B')
('A', 'C')
('B', 'A')
('B', 'B')
('B', 'C')
('C', 'A')
('C', 'B')
('C', 'C')
提问:既然可以用嵌套的for
循环来代替,为什么还要封装该函数?
解答:一两个for
循环可以,但如果你需要写七八个for
循环,代码就很难看了,所以一般都会用该函数替换嵌套的for
循环。
该函数可以应用到密码字典的生成中,比如生成一个包含所有由数字组成的长度为8的密码字典:
import itertools as its
iterator = its.product('0123456789', repeat=8)
with open('pwd.txt', 'w') as f:
for i in iterator:
f.write(''.join(i)+'\n')
4.2. permutations(p, [r])
同product()
函数,但元素不允许重复,不会生成AA
这样的:
>>> import itertools as its
>>> a = its.permutations('ABC', r=2)
>>> for i in a:
... print(i)
...
('A', 'B')
('A', 'C')
('B', 'A')
('B', 'C')
('C', 'A')
('C', 'B')
4.3. combinations(p, r)
同permutations()
函数,但AB
和BA
这样的视为同一元素,只取AB:
>>> import itertools as its
>>> a = its.combinations('ABC', r=2)
>>> for i in a:
... print(i)
...
('A', 'B')
('A', 'C')
('B', 'C')
4.4. combinations_with_replacement(p, r)
同product()
函数,允许重复,但AB
和BA
这样的视为同一元素,只取AB
:
>>> import itertools as its
>>> a = its.combinations_with_replacement('ABC', r=2)
>>> for i in a:
... print(i)
...
('A', 'A')
('A', 'B')
('A', 'C')
('B', 'B')
('B', 'C')
('C', 'C')
5. 引用
https://docs.python.org/3.10/library/itertools.html#itertools-recipes