What is the concept of Python collections? Introduction to Python Collections

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Description of collection

A collection is an unordered, non-duplicate data collection. It itself is mutable, but the elements inside are immutable. Type (note here that even a tuple containing a list is not allowed, it must be a completely immutable type)

Create a collection

You can use curly brackets to create a collection {} is used to create, and the elements are separated by commas, but it cannot be used to create an empty collection, because {} creates an empty dictionary.

如 set1 = {1,2,3}

You can also use the set() function to create a collection, where the parameters are iterable objects (such as strings, lists, tuples, iterators, etc.), provided that the elements must be immutable type.

i = 'abcdefg'
a =set(i)
print(type(a))
print(a)
----------
<class>
{'b', 'a', 'f', 'd', 'g', 'e', 'c'}</class>

Add add update

add(elem)
You can add a new element to the collection, if the element already exists, it will not Make any changes

s1 = {1,2,3}
s1.add('qwe')
print(s1)
----------
{1, 2, 3, 'qwe'}

update(*others)
Can add other iterable objects (such as strings, lists, tuples) to the collection and automatically remove duplicate elements

s1 = {1,2,3}
it = [3,4,5]
s1.update(it)
print(s1)
----------
{1, 2, 3, 4, 5}

Delete remove(elem) discard(elem) pop() clear()

remove(elem)
Remove elements from the collection If the element does not exist, an error will be reported.

s1 = {1,2,3}
s1.remove(2)
print(s1)
----------
{1, 3}

discard(elem)
Like remove, you can delete elements from the collection. The difference is that if the element does not exist, no error will be reported.

s1 = {1,2,3}
s1.discard(6)
print(s1)
----------
{1, 2, 3}

pop()
Randomly delete an element from the set and return the element. If the set is empty, an error will be reported

s1 = {1,2,3}
a = s1.pop()
print(s1)
print(a)
----------
{2, 3}
1

clear ()
is used to clear the collection.

Several operating methods

There are several operating methods for sets: intersection, union, difference set, anti-intersection, subset, superset
The following uses code to explain the usage of each method in detail

s1 = {1,2,3,4,5}
s2 = {3,4,5,6,7}
print('交集：',s1 & s2) #可用&或者intersection，输出一个新的集合，包含共同拥有的元素
print('交集：',s1.intersection(s2))
print('并集：',s1 | s2) #可用|或者union，输出一个新的集合，包含两个集合中所有的元素（去重）
print('并集：',s1.union(s2))
print('差集：',s1 - s2) #可用 - 或者difference,输出一个新的集合，包含前一个集合中除去共有的元素
print('差集：',s1.difference(s2))
print('反交集：',s1 ^ s2) #可用^或者 symmetric_difference,输出一个新的集合，包含两个集合中除去共有的元素后剩余的所有元素
print('反交集：',s1.symmetric_difference(s2))

    交集： {3, 4, 5}
    交集： {3, 4, 5}
    并集： {1, 2, 3, 4, 5, 6, 7}
    并集： {1, 2, 3, 4, 5, 6, 7}
    差集： {1, 2}
    差集： {1, 2}
    反交集： {1, 2, 6, 7}
    反交集： {1, 2, 6, 7}

Subsets and supersets. Simply put, the elements in one set contain all the elements in another set. The "large" set is called Superset, "small" is called subset

s1 = {1,2,3}
s2 = {1,2,3,4,5}
print(s1.issubset(s2)) #输出为True，s1是s2的子集
print(s2.issuperset(s1))#输出为True，s2是s1的超集
----------
    True
    True

In addition, using frozenset('set name'), you can turn the set into an immutable set

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