Python内置库itertools简单学习

该库为满足特定需要的比较高效的迭代器内置库，在数据科学中的应用也不少，故有必要了解一下：

import itertools

import sys

无限迭代器(Infinite iterators)

Iterator	Arguments	Results	Example
count()	start, [step]	start, start+step, start+2*step, …	count(10) --> 10 11 12 13 14 ...
cycle()	p	p0, p1, … plast, p0, p1, …	cycle('ABCD') --> A B C D A B C D ...
repeat()	elem [,n]	elem, elem, elem, … endlessly or up to n times	repeat(10, 3) --> 10 10 10

• itertools.count

a=itertools.count(5)

type(a)

itertools.count

next(a)

5

next(a)

6

next(a)

7

b=itertools.count(6,2)

next(b)

6

next(b)

8

• itertools.cycle

a=itertools.cycle([1,2])

next(a)

1

next(a)

2

next(a)

1

next(a)

2

b=itertools.cycle('ABC')

next(b)

'A'

next(b)

'B'

next(b)

'C'

next(b)

'A'

• itertools.repeat

a=itertools.repeat([1,2,3])

next(a)

[1, 2, 3]

next(a)

[1, 2, 3]

b=itertools.repeat('ABC',3)

next(b)

'ABC'

next(b)

'ABC'

next(b)

'ABC'

try:
    next(b)
except Exception as e:
    print(sys.exc_info())

(<class 'StopIteration'>, StopIteration(), <traceback object at 0x0000024CB3EE2E08>)

最短输入停止迭代器（Iterators terminating on the shortest input sequence）

Iterator	Arguments	Results	Example
accumulate()	p [,func]	p0, p0+p1, p0+p1+p2, …	accumulate([1,2,3,4,5]) --> 1 3 6 10 15
chain()	p, q, …	p0, p1, … plast, q0, q1, …	chain('ABC', 'DEF') --> A B C D E F
chain.from_iterable()	iterable	p0, p1, … plast, q0, q1, …	chain.from_iterable(['ABC', 'DEF']) --> A B C D E F
compress()	data, selectors	(d[0] if s[0]), (d[1] if s[1]), …	compress('ABCDEF', [1,0,1,0,1,1]) --> A C E F
dropwhile()	pred, seq	seq[n], seq[n+1], starting when pred fails	dropwhile(lambda x: x<5, [1,4,6,4,1]) --> 6 4 1
filterfalse()	pred, seq	elements of seq where pred(elem) is false	filterfalse(lambda x: x%2, range(10)) --> 0 2 4 6 8
groupby()	iterable[, key]	sub-iterators grouped by value of key(v)
islice()	seq, [start,] stop [, step]	elements from seq[start:stop:step]	islice('ABCDEFG', 2, None) --> C D E F G
starmap()	func, seq	func(seq[0]), func(seq[1]), …	starmap(pow, [(2,5), (3,2), (10,3)]) --> 32 9 1000
takewhile()	pred, seq	seq[0], seq[1], until pred fails	takewhile(lambda x: x<5, [1,4,6,4,1]) --> 1 4
tee()	it, n	it1, it2, … itn splits one iterator into n
zip_longest()	p, q, …	(p[0], q[0]), (p[1], q[1]), …	zip_longest('ABCD', 'xy', fillvalue='-') --> Ax By C- D-

• itertools.accumulate

a=itertools.accumulate([1,2,3,4])

list(a)

[1, 3, 6, 10]

b=itertools.accumulate('abc')

list(b)

['a', 'ab', 'abc']

• itertools.chain

a=itertools.chain([1,2],[3,4],[5,6])

list(a)

[1, 2, 3, 4, 5, 6]

b=itertools.chain('ab','cd')

list(b)

['a', 'b', 'c', 'd']

• itertools.chain.from_iterable

a=itertools.chain.from_iterable(['abc','def'])

list(a)

['a', 'b', 'c', 'd', 'e', 'f']

• itertools.compress

a=itertools.compress('abcdef',[1,0,1,0,1,1])

list(a)

['a', 'c', 'e', 'f']

a=itertools.compress('abcdef',[1,False,1,False,1,0])

list(a)

['a', 'c', 'e']

• itertools.dropwhile

a=itertools.dropwhile(lambda x:x<5,[1,4,6,4,1])

list(a)

[6, 4, 1]

• itertools.takewhile

a=itertools.takewhile(lambda x:x<5,[1,4,6,4,1])

list(a)

[1, 4]

• filterfalse

a=itertools.filterfalse(lambda x:x%2,range(10))

list(a)

[0, 2, 4, 6, 8]

• groupby

x = [(1, 2), (2, 3), (1, 4), (5, 5), (3, 4), (2, 6)]

a=itertools.groupby(x,lambda x:x[0])

for i in a:
    print(i[0],list(i[1]))

1 [(1, 2)]
2 [(2, 3)]
1 [(1, 4)]
5 [(5, 5)]
3 [(3, 4)]
2 [(2, 6)]

• itertools.islice

a=itertools.islice('abcdefg',2,None)

list(a)

['c', 'd', 'e', 'f', 'g']

b=itertools.islice('abcdefg',2,4)

list(b)

['c', 'd']

• itertools.starmap

a=itertools.starmap(pow,[(2,5),(3,2)])

list(a)

[32, 9]

• itertools.tee

a=itertools.tee([1,2,3],2)

list(a)

[<itertools._tee at 0x24cb3f29448>, <itertools._tee at 0x24cb3f29888>]

a=itertools.tee([1,2,3],2)

list(a[0])

[1, 2, 3]

list(a[1])

[1, 2, 3]

• itertools.zip_longest

a=itertools.zip_longest('abcd','xy',fillvalue='-')

list(a)

[('a', 'x'), ('b', 'y'), ('c', '-'), ('d', '-')]

组合迭代器

Iterator	Arguments	Results
product()	p, q, … [repeat=1]	cartesian product, equivalent to a nested for-loop
permutations()	p[, r]	r-length tuples, all possible orderings, no repeated elements
combinations()	p, r	r-length tuples, in sorted order, no repeated elements
combinations_with_replacement()	p, r	r-length tuples, in sorted order, with repeated elements
product('ABCD', repeat=2)		AA AB AC AD BA BB BC BD CA CB CC CD DA DB DC DD
permutations('ABCD', 2)		AB AC AD BA BC BD CA CB CD DA DB DC
combinations('ABCD', 2)		AB AC AD BC BD CD
combinations_with_replacement('ABCD', 2)		AA AB AC AD BB BC BD CC CD DD

• itertools.product

a=itertools.product([1,2,3],[4,5,6])

list(a)

[(1, 4), (1, 5), (1, 6), (2, 4), (2, 5), (2, 6), (3, 4), (3, 5), (3, 6)]

a=itertools.product([1,2,3],[4,5,6],repeat=2)

list(a)

[(1, 4, 1, 4),
(1, 4, 1, 5),
(1, 4, 1, 6),
(1, 4, 2, 4),
(1, 4, 2, 5),
(1, 4, 2, 6),
(1, 4, 3, 4),
(1, 4, 3, 5),
(1, 4, 3, 6),
(1, 5, 1, 4),
(1, 5, 1, 5),
(1, 5, 1, 6),
(1, 5, 2, 4),
(1, 5, 2, 5),
(1, 5, 2, 6),
(1, 5, 3, 4),
(1, 5, 3, 5),
(1, 5, 3, 6),
(1, 6, 1, 4),
(1, 6, 1, 5),
(1, 6, 1, 6),
(1, 6, 2, 4),
(1, 6, 2, 5),
(1, 6, 2, 6),
(1, 6, 3, 4),
(1, 6, 3, 5),
(1, 6, 3, 6),
(2, 4, 1, 4),
(2, 4, 1, 5),
(2, 4, 1, 6),
(2, 4, 2, 4),
(2, 4, 2, 5),
(2, 4, 2, 6),
(2, 4, 3, 4),
(2, 4, 3, 5),
(2, 4, 3, 6),
(2, 5, 1, 4),
(2, 5, 1, 5),
(2, 5, 1, 6),
(2, 5, 2, 4),
(2, 5, 2, 5),
(2, 5, 2, 6),
(2, 5, 3, 4),
(2, 5, 3, 5),
(2, 5, 3, 6),
(2, 6, 1, 4),
(2, 6, 1, 5),
(2, 6, 1, 6),
(2, 6, 2, 4),
(2, 6, 2, 5),
(2, 6, 2, 6),
(2, 6, 3, 4),
(2, 6, 3, 5),
(2, 6, 3, 6),
(3, 4, 1, 4),
(3, 4, 1, 5),
(3, 4, 1, 6),
(3, 4, 2, 4),
(3, 4, 2, 5),
(3, 4, 2, 6),
(3, 4, 3, 4),
(3, 4, 3, 5),
(3, 4, 3, 6),
(3, 5, 1, 4),
(3, 5, 1, 5),
(3, 5, 1, 6),
(3, 5, 2, 4),
(3, 5, 2, 5),
(3, 5, 2, 6),
(3, 5, 3, 4),
(3, 5, 3, 5),
(3, 5, 3, 6),
(3, 6, 1, 4),
(3, 6, 1, 5),
(3, 6, 1, 6),
(3, 6, 2, 4),
(3, 6, 2, 5),
(3, 6, 2, 6),
(3, 6, 3, 4),
(3, 6, 3, 5),
(3, 6, 3, 6)]

• itertools.permutations

for i in itertools.permutations([1,2,3],2):
    print (i)

(1, 2)
(1, 3)
(2, 1)
(2, 3)
(3, 1)
(3, 2)

for i in itertools.permutations('123',2):
    print (i)

('1', '2')
('1', '3')
('2', '1')
('2', '3')
('3', '1')
('3', '2')

for i in itertools.permutations('122',2):
    print (i)

('1', '2')
('1', '2')
('2', '1')
('2', '2')
('2', '1')
('2', '2')

• itertools.combinations

a=itertools.combinations([1,2,3],2)

list(a)

[(1, 2), (1, 3), (2, 3)]

a=itertools.combinations('abcd',2)

list(a)

[('a', 'b'), ('a', 'c'), ('a', 'd'), ('b', 'c'), ('b', 'd'), ('c', 'd')]

• itertools.combinations_with_replacement

a=itertools.combinations_with_replacement('abcd',2)

list(a)

[('a', 'a'),
('a', 'b'),
('a', 'c'),
('a', 'd'),
('b', 'b'),
('b', 'c'),
('b', 'd'),
('c', 'c'),
('c', 'd'),
('d', 'd')]