8.7. sets — 唯一元素的无序集合 ¶

2.3 版新增。

Deprecated since version 2.6: 内置 set / frozenset types replace this module.

The sets module provides classes for constructing and manipulating unordered collections of unique elements. Common uses include membership testing, removing duplicates from a sequence, and computing standard math operations on sets such as intersection, union, difference, and symmetric difference.

Like other collections, sets support x in set , len(set) ，和 for x in set . Being an unordered collection, sets do not record element position or order of insertion. Accordingly, sets do not support indexing, slicing, or other sequence-like behavior.

Most set applications use the Set class which provides every set method except for __hash__() . For advanced applications requiring a hash method, the ImmutableSet class adds a __hash__() method but omits methods which alter the contents of the set. Both Set and ImmutableSet derive from BaseSet , an abstract class useful for determining whether something is a set: isinstance(obj, BaseSet) .

The set classes are implemented using dictionaries. Accordingly, the requirements for set elements are the same as those for dictionary keys; namely, that the element defines both __eq__() and __hash__() . As a result, sets cannot contain mutable elements such as lists or dictionaries. However, they can contain immutable collections such as tuples or instances of ImmutableSet . For convenience in implementing sets of sets, inner sets are automatically converted to immutable form, for example, Set([Set(['dog'])]) is transformed to Set([ImmutableSet(['dog'])]) .

class sets. Set ( [ iterable ] ) ¶

Constructs a new empty Set object. If the optional iterable parameter is supplied, updates the set with elements obtained from iteration. All of the elements in iterable should be immutable or be transformable to an immutable using the protocol described in section Protocol for automatic conversion to immutable .

class sets. ImmutableSet ( [ iterable ] ) ¶

Constructs a new empty ImmutableSet object. If the optional iterable parameter is supplied, updates the set with elements obtained from iteration. All of the elements in iterable should be immutable or be transformable to an immutable using the protocol described in section Protocol for automatic conversion to immutable .

因为 ImmutableSet objects provide a __hash__() method, they can be used as set elements or as dictionary keys. ImmutableSet objects do not have methods for adding or removing elements, so all of the elements must be known when the constructor is called.

8.7.1. 集对象 ¶

实例化的 Set and ImmutableSet both provide the following operations:

注意，非运算符版本的 union() , intersection() , difference() ，和 symmetric_difference() will accept any iterable as an argument. In contrast, their operator based counterparts require their arguments to be sets. This precludes error-prone constructions like Set('abc') & 'cbs' in favor of the more readable Set('abc').intersection('cbs') .

Changed in version 2.3.1: Formerly all arguments were required to be sets.

In addition, both Set and ImmutableSet support set to set comparisons. Two sets are equal if and only if every element of each set is contained in the other (each is a subset of the other). A set is less than another set if and only if the first set is a proper subset of the second set (is a subset, but is not equal). A set is greater than another set if and only if the first set is a proper superset of the second set (is a superset, but is not equal).

The subset and equality comparisons do not generalize to a complete ordering function. For example, any two disjoint sets are not equal and are not subsets of each other, so all of the following return False : a<b , a==b ，或 a>b . Accordingly, sets do not implement the __cmp__() 方法。

Since sets only define partial ordering (subset relationships), the output of the list.sort() method is undefined for lists of sets.

The following table lists operations available in ImmutableSet but not found in Set :

The following table lists operations available in Set but not found in ImmutableSet :

注意，非运算符版本的 update() , intersection_update() , difference_update() ，和 symmetric_difference_update() will accept any iterable as an argument.

Changed in version 2.3.1: Formerly all arguments were required to be sets.

Also note, the module also includes a union_update() method which is an alias for update() . The method is included for backwards compatibility. Programmers should prefer the update() method because it is supported by the built-in set() and frozenset() 类型。

8.7.2. 范例 ¶

>>> from sets import Set
>>> engineers = Set(['John', 'Jane', 'Jack', 'Janice'])
>>> programmers = Set(['Jack', 'Sam', 'Susan', 'Janice'])
>>> managers = Set(['Jane', 'Jack', 'Susan', 'Zack'])
>>> employees = engineers | programmers | managers           # union
>>> engineering_management = engineers & managers            # intersection
>>> fulltime_management = managers - engineers - programmers # difference
>>> engineers.add('Marvin')                                  # add element
>>> print engineers
Set(['Jane', 'Marvin', 'Janice', 'John', 'Jack'])
>>> employees.issuperset(engineers)     # superset test
False
>>> employees.update(engineers)         # update from another set
>>> employees.issuperset(engineers)
True
>>> for group in [engineers, programmers, managers, employees]:
...     group.discard('Susan')          # unconditionally remove element
...     print group
...
Set(['Jane', 'Marvin', 'Janice', 'John', 'Jack'])
Set(['Janice', 'Jack', 'Sam'])
Set(['Jane', 'Zack', 'Jack'])
Set(['Jack', 'Sam', 'Jane', 'Marvin', 'Janice', 'John', 'Zack'])