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• Python frozenset aggregate

Python frozenset aggregate

set A set is a variable sequence , The program can change the elements in the sequence ;frozenset A set is an immutable sequence , The program cannot change the elements in the sequence .set All the methods in a set that can change the set itself , such as remove()、discard()、add() etc. ,frozenset Don't support ;set A method in a set that does not change the set itself ,fronzenset All support .

We can enter... In an interactive programming environment dir(frozenset) Check it out. frozenset Collection support methods ：

>>> dir(frozenset)
['copy', 'difference', 'intersection', 'isdisjoint', 'issubset', 'issuperset', 'symmetric_difference', 'union']

frozenset These methods and methods of collection set The function of the method with the same name in the collection is the same .

In two cases, you can use fronzenset：

1. When the elements of the set do not need to be changed , We can use fronzenset replace set, It's safer .
2. Sometimes programs require immutable objects , Also use this time fronzenset replace set. such as , Dictionaries （dict） Key （key） It is required to be an immutable object .

The following program demonstrates frozenset Usage of ：

s = {
'Python', 'C', 'C++'}
fs = frozenset(['Java', 'Shell'])
s_sub = {
'PHP', 'C#'}
# towards set Add... To the collection frozenset
s.add(fs)
print('s =', s)
# for set Add child to collection set aggregate 
s.add(s_sub)
print('s =', s)

Running results ：

s = {
'Python', frozenset({
'Java', 'Shell'}), 'C', 'C++'}
Traceback (most recent call last):
File "C:\Users\mozhiyan\Desktop\demo.py", line 11, in <module>
s.add(s_sub)
TypeError: unhashable type: 'set'

It should be noted that ,set The elements of the collection itself must be immutable , therefore set The element of cannot be set, Can only be frozenset. The first 6 Line code to set Add frozenset No problem , because frozenset It's immutable ; however , The first 10 In line of code, try to set Add to the cake set, This is not allowed , because set Is variable .