python 集合类型
There are two types of set:variable set and immutable set：variable set can not be hashed.Note:frozenset can not be changed!

1.create and update

> myset=set("abadsfljasd")
> myset
set(['a', 'b', 'd', 'f', 'j', 'l', 's'])

Note:we can not create a set by operator（can not treated as list ant tuple or diretoty）.we can only use set() anf frozenzet().

1.1visit the value in a set:

'a' in myset

1.2update a set

myset.add('z')
myset.update("python")
myset.remove("abcd")
s-=set("python")

1.3delete a set

del myset

2.the oprator of set

2.1 standard operator

in,no in,==,!= , > , >=, <, <=

2.2 'set' type operator

combine: "|"
> twoset=se("xyz")
> oneset | twoset
set(['a', 'c', 'b', 'd', 'y', 'x', 'z'])

&, -, ^

>oneset ^ threeset
>set(['c', 'd', '1', '3', '2'])

mixed-type operate:set and frozen set
> myset=set("abcd")
> myfrozenset=frozenset("1234")
> myset | myfrozenset
set(['a', 'c', 'b', 'd', '1', '3', '2', '4'])
> myfrozenset | myset
frozenset(['a', 'c', 'b', 'd', '1', '3', '2', '4'])

Note:the result type follow the former.

2.3set-type operator---only used for set not for frozenset

Update:|=, &=, -=, ^=

3.build_in function

len(), set(), frozenset()

3.1 set-type build_in function

Note:build_in function means standart build_in function that could used for set .set-type build_in function means this function is part of set-type.

function name           return value

s.add('a')              update s
s.clear()               clear s
s.difference(t)         a new set(element in s but not in t)
s.union(t)              a new set
s.intersection(t)       a new set
s.issubset(t)           True or False

3.2compare: operator and build_in function:

For the most, build_in function and operarot is equal. However, the operand for the operator must be set-type, while the object for the build_in function could be iterative type.