8.7. "sets" — Unordered collections of unique elements
******************************************************

2.3 版新加入.

2.6 版後已棄用: The built-in "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)", and "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.

   Because "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. Set Objects
==================

Instances of "Set" and "ImmutableSet" both provide the following
operations:

+---------------------------------+--------------+-----------------------------------+
| Operation                       | Equivalent   | Result                            |
+=================================+==============+===================================+
| "len(s)"                        |              | number of elements in set *s*     |
|                                 |              | (cardinality)                     |
+---------------------------------+--------------+-----------------------------------+
| "x in s"                        |              | test *x* for membership in *s*    |
+---------------------------------+--------------+-----------------------------------+
| "x not in s"                    |              | test *x* for non-membership in    |
|                                 |              | *s*                               |
+---------------------------------+--------------+-----------------------------------+
| "s.issubset(t)"                 | "s <= t"     | test whether every element in *s* |
|                                 |              | is in *t*                         |
+---------------------------------+--------------+-----------------------------------+
| "s.issuperset(t)"               | "s >= t"     | test whether every element in *t* |
|                                 |              | is in *s*                         |
+---------------------------------+--------------+-----------------------------------+
| "s.union(t)"                    | "s | t"      | new set with elements from both   |
|                                 |              | *s* and *t*                       |
+---------------------------------+--------------+-----------------------------------+
| "s.intersection(t)"             | "s & t"      | new set with elements common to   |
|                                 |              | *s* and *t*                       |
+---------------------------------+--------------+-----------------------------------+
| "s.difference(t)"               | "s - t"      | new set with elements in *s* but  |
|                                 |              | not in *t*                        |
+---------------------------------+--------------+-----------------------------------+
| "s.symmetric_difference(t)"     | "s ^ t"      | new set with elements in either   |
|                                 |              | *s* or *t* but not both           |
+---------------------------------+--------------+-----------------------------------+
| "s.copy()"                      |              | new set with a shallow copy of    |
|                                 |              | *s*                               |
+---------------------------------+--------------+-----------------------------------+

Note, the non-operator versions of "union()", "intersection()",
"difference()", and "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')".

2.3.1 版更變: 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", or "a>b". Accordingly, sets do not implement
the "__cmp__()" method.

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":

+---------------+--------------------------------+
| Operation     | Result                         |
+===============+================================+
| "hash(s)"     | returns a hash value for *s*   |
+---------------+--------------------------------+

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

+----------------------------------------+---------------+-----------------------------------+
| Operation                              | Equivalent    | Result                            |
+========================================+===============+===================================+
| "s.update(t)"                          | *s* |= *t*    | return set *s* with elements      |
|                                        |               | added from *t*                    |
+----------------------------------------+---------------+-----------------------------------+
| "s.intersection_update(t)"             | *s* &= *t*    | return set *s* keeping only       |
|                                        |               | elements also found in *t*        |
+----------------------------------------+---------------+-----------------------------------+
| "s.difference_update(t)"               | *s* -= *t*    | return set *s* after removing     |
|                                        |               | elements found in *t*             |
+----------------------------------------+---------------+-----------------------------------+
| "s.symmetric_difference_update(t)"     | *s* ^= *t*    | return set *s* with elements from |
|                                        |               | *s* or *t* but not both           |
+----------------------------------------+---------------+-----------------------------------+
| "s.add(x)"                             |               | add element *x* to set *s*        |
+----------------------------------------+---------------+-----------------------------------+
| "s.remove(x)"                          |               | remove *x* from set *s*; raises   |
|                                        |               | "KeyError" if not present         |
+----------------------------------------+---------------+-----------------------------------+
| "s.discard(x)"                         |               | removes *x* from set *s* if       |
|                                        |               | present                           |
+----------------------------------------+---------------+-----------------------------------+
| "s.pop()"                              |               | remove and return an arbitrary    |
|                                        |               | element from *s*; raises          |
|                                        |               | "KeyError" if empty               |
+----------------------------------------+---------------+-----------------------------------+
| "s.clear()"                            |               | remove all elements from set *s*  |
+----------------------------------------+---------------+-----------------------------------+

Note, the non-operator versions of "update()",
"intersection_update()", "difference_update()", and
"symmetric_difference_update()" will accept any iterable as an
argument.

2.3.1 版更變: 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()" types.


8.7.2. Example
==============

>>> 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 # doctest: +SKIP
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]: # doctest: +SKIP
...     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'])


8.7.3. Protocol for automatic conversion to immutable
=====================================================

Sets can only contain immutable elements.  For convenience, mutable
"Set" objects are automatically copied to an "ImmutableSet" before
being added as a set element.

The mechanism is to always add a *hashable* element, or if it is not
hashable, the element is checked to see if it has an
"__as_immutable__()" method which returns an immutable equivalent.

Since "Set" objects have a "__as_immutable__()" method returning an
instance of "ImmutableSet", it is possible to construct sets of sets.

A similar mechanism is needed by the "__contains__()" and "remove()"
methods which need to hash an element to check for membership in a
set.  Those methods check an element for hashability and, if not,
check for a "__as_temporarily_immutable__()" method which returns the
element wrapped by a class that provides temporary methods for
"__hash__()", "__eq__()", and "__ne__()".

The alternate mechanism spares the need to build a separate copy of
the original mutable object.

"Set" objects implement the "__as_temporarily_immutable__()" method
which returns the "Set" object wrapped by a new class
"_TemporarilyImmutableSet".

The two mechanisms for adding hashability are normally invisible to
the user; however, a conflict can arise in a multi-threaded
environment where one thread is updating a set while another has
temporarily wrapped it in "_TemporarilyImmutableSet".  In other words,
sets of mutable sets are not thread-safe.


8.7.4. Comparison to the built-in "set" types
=============================================

The built-in "set" and "frozenset" types were designed based on
lessons learned from the "sets" module.  The key differences are:

* "Set" and "ImmutableSet" were renamed to "set" and "frozenset".

* There is no equivalent to "BaseSet".  Instead, use "isinstance(x,
  (set, frozenset))".

* The hash algorithm for the built-ins performs significantly better
  (fewer collisions) for most datasets.

* The built-in versions have more space efficient pickles.

* The built-in versions do not have a "union_update()" method.
  Instead, use the "update()" method which is equivalent.

* The built-in versions do not have a "_repr(sorted=True)" method.
  Instead, use the built-in "repr()" and "sorted()" functions:
  "repr(sorted(s))".

* The built-in version does not have a protocol for automatic
  conversion to immutable.  Many found this feature to be confusing
  and no one in the community reported having found real uses for it.
