How to use the itertools module of Python standard library

Introduction

Official description: Functional tools for creating and using iterators. That is, functions for creating efficient iterators.

itertools.chain(*iterable)

Return multiple sequences as a single sequence.
For example:

import itertools
for each in itertools.chain('i', 'love', 'python'):
    print each

Output:

i
l
o
v
e
p
y
t
h
o
n

itertools.combinations(iterable, r)

Returns "combinations" of the specified length
For example:

import itertools
for each in itertools.combinations('abc', 2):
    print each

Output:

('a', 'b')
('a', 'c')
('b', 'c')

itertools.combinations_with_replacement(iterable, r)

Returns a "combination" of the specified length, and the elements in the combination can be repeated
For example:

import itertools
for each in itertools.combinations_with_replacement('abc', 2):
    print each

Output:

('a', 'a')
('a', 'b')
('a', 'c')
('b', 'b')
('b', 'c')
('c', 'c')

itertools.product(*iterable[,repeat])

Returns all combinations of the specified length, which can be understood as the Cartesian product
For example:

import itertools
for each in itertools.product('abc', repeat=2):
    print each

('a', 'a')
('a', 'b')
('a', 'c')
('b', 'a')
('b', 'b')
('b', 'c')
('c', 'a')
('c', 'b')
( 'c', 'c')

itertools.premutations(iteravle[,r])

Return a permutation of length r
For example:

import itertools
for value in itertools.permutations('abc', 2):
    print value

Output:

('a', 'b')
('a', 'c')
('b', 'a')
('b', 'c')
('c', 'a')
('c', 'b')

itertools.compress(data,selector)

Returns the corresponding element of data whose selector is True
For example:

import itertools
for each in itertools.compress('abcd', [1, 0, 1, 0]):
    print each

Output:

a
c

itertools.count(start=0, step=1)

Returns a sequence starting with start and increasing step, infinitely increasing
For example:

import itertools
for each in itertools.count(start=0, step=2):
    print each

Output:

1
2
3
.
.

itertools.cycle(iterable)

Iterate the iterator infinitely
For example:

import itertools
for each in itertools.cycle('ab'):
    print each

Output:

a
b
a
b
.
.

itertools.dropwhile(predicate, iterable)

Until predicate is true, return iterable subsequent data, otherwise drop it
For example:

import itertools
for each in itertools.dropwhile(lambda x: x<5, [2,1,6,8,2,1]):
    print each

Output:

6
8
2
1

itertools.groupby(iterable[,key])

Return a group (key, itera), key is the value of iterable, itera is all items equal to key
For example:

import itertools
for key, vale in itertools.groupby('aabbbc'):
    print key, list(vale)

Output:

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

itertools. ifilter(predicate, iterable)

Returns an element iterator whose predicate result is True. If predicate is None, returns all items that are True in iterable
For example:

import itertools
for value in itertools.ifilter(lambda x: x % 2, range(10)):
    print value

Output:

1
3
5
7
9

itertools.ifilterfasle(predicate,iterable)

Returns elements whose predicate is False. If predicate is None, returns all items in iterable that are False.
For example:

import itertools
for value in itertools.ifilterfalse(lambda x: x % 2, range(10)):
    print value

Output:

0
2
4
6
8

itertools.imap(function,*iterables)

Equivalent to map() in iterator mode
For example:

import itertools
for value in itertools.imap(lambda x, y: x+y, (1,2,3), (4,5,6)):
    print value

Output :

5
7
9

itertools.islice(iterable, start,stop[,step])

Equivalent to iterator mode slicing operation
For example:

import itertools
for value in itertools.islice('abcdefg', 1, 4, 2):
    print value

Output:

b
d

itertools.repeat(object,[,times])

Continuously return the object object. If times is specified, return times times
For example:

import itertools
for value in itertools.repeat('a', 2):
    print value

Output:

a
a

itertools.starmap(function,iterable)

Returns the value of function(iter), iter is the element of iterable
For example:

import itertools
for value in itertools.starmap(lambda x, y: x * y, [(1, 2), (3, 4)]):
    print value

Output:

2
12

itertools.takewhile(predicate,iterable)

If predicate is true, return the iterable element, if it is false, it will not return, break.
For example:

import itertools
for value in itertools.takewhile(lambda x: x < 5, [1, 3, 5, 6]):
    print value

Output:

1
3

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