In-depth understanding of the itertools module in Python

Python Video Tutorial column introduces the itertools module.

There is a powerful iteration tool package itertools in Python, which is one of the standard tool packages that comes with Python.

product

Since itertools is a built-in library, no installation is required, just import itertools.

product is used to find the Cartesian product of multiple iterable objects(Cartesian Product), which is equivalent to a nested for loop. That is:

Cartesian The product refers to the Cartesian product (Cartesian product) of two sets X and Y in mathematics, also known as the direct product, expressed as X × Y.

product(A, B) is the same as ``((x,y) for x in A for y in B)`.

import itertools
for item in itertools.product([1,2,3],[100,200]):
    print(item)
    
    
# 输出如下
(1, 100)
(1, 200)
(2, 100)
(2, 200)
(3, 100)
(3, 200)复制代码

permutations

In layman's terms, permutations return the full arrangement of all mathematics or characters of an iterable object.

Full permutation, that is, all permutations that produce a specified number of elements (depending on the order), which is the A in the high school permutation combination.

permutationsIt accepts a collection object and then produces a sequence of tuples.

For example, print(list(itertools.permutations('abc',3))), a total of $A_3^3=6$

items = ['a','b','c']
from itertools import permutations
for i in permutations(items):
    print(i) #排列组合

print(list(itertools.permutations('abc',3))) 
# 输出如下
('a', 'b', 'c')
('a', 'c', 'b')
('b', 'a', 'c')
('b', 'c', 'a')
('c', 'a', 'b')
('c', 'b', 'a')
[('a', 'b', 'c'), ('a', 'c', 'b'), ('b', 'a', 'c'), ('b', 'c', 'a'), ('c', 'a', 'b'), ('c', 'b', 'a')]复制代码

If you need all permutations of a specified length, you can pass an optional length parameter r.

items = ['a','b','c']
from itertools import permutations
for i in permutations(items,2):
    print(i) #排列组合
    
# 输出如下
('a', 'b')
('a', 'c')
('b', 'a')
('b', 'c')
('c', 'a')
('c', 'b')复制代码

combinationsFind all combinations in which the specified number of elements in the list or generator are not repeated

itertools.permutations(iter,r)

and The difference between

itertools.combinations(iter,r)

is: the former permutations allows reuse, while the latter combinations cannot be reused<pre class="brush:php;toolbar:false">&gt;&gt;&gt; print(list(itertools.combinations('abc',3))) [('a', 'b', 'c')]复制代码</pre>combinations_with_replacement

combinations_with_replacement

is very similar to

combinations

, the only difference is that the formercombinations_with_replacementThe data in the collection type can be repeated <pre class="brush:php;toolbar:false">&gt;&gt;&gt; print(list(itertools.combinations_with_replacement('abc',3))) [('a', 'a', 'a'), ('a', 'a', 'b'), ('a', 'a', 'c'), ('a', 'b', 'b'), ('a', 'b', 'c'), ('a', 'c', 'c'), ('b', 'b', 'b'), ('b', 'b', 'c'), ('b', 'c', 'c'), ('c', 'c', 'c')]复制代码</pre>accumulate

accumulate

is used to accumulate elements in the list one by one

>>> import itertools
>>> x = itertools.accumulate(range(10))
>>> print(list(x))
[0, 1, 3, 6, 10, 15, 21, 28, 36, 45]复制代码

compress

compress()

Yes Filter tool, which accepts an iterable object and a Boolean selection sequence as input, and outputs all iterable objects that are True in the Boolean sequence.

import itertools

its=["a","b","c","d","e","f","g","h"]
selector=[True,False,1,0,3,False,-2,"y"]
for item in itertools.compress(its,selector):
    print (item)
    
a
c
e
g
h   
复制代码

count

count(initial value=0, step=1)

is to create an iterator, evenly spaced starting from the passed in starting parameter numerical value.

Let’s look at a simple example<pre class="brush:php;toolbar:false">from itertools import count for i in count(10): #从10开始无限循环     if i &gt; 20:          break     else:         print(i) 10 11 12 13 14 15 16 17 18 19 20复制代码</pre>chain

chain chain is mainly used to connect multiple sequences together for iteration.

import itertools
chain = itertools.chain([1, 2, 3], [4, 5, 6])
for c in chain:
   print(c)
1
2
3
4
5
6  
复制代码

Another very important function of chain is to flatten the list.

>>> list(itertools.chain([1, 2, 3], [4, 5], [6] ,[7,8]))
[1, 2, 3, 4, 5, 6, 7, 8]复制代码

cycle

import itertools
cycle = itertools.cycle([1, 2, 3])
for c in cycle:
   print(c)复制代码

The operation result output is 1 2 3 1 2 3... It keeps going round and round, never stopping.

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