python的set和其他语言类似, 是一个无序不重复元素集, 基本功能包括关系测试和消除重复元素. 集合对象还支持union(联合), intersection(交), difference(差)和sysmmetric difference(对称差集)等数易做图算.
sets 支持 x in set, len(set),和 for x in set。作为一个无序的集合，sets不记录元素位置或者插入点。因此，sets不支持 indexing, slicing, 或其它类序列（sequence-like）的操作。
Help on class set in module __builtin__:
class set(object)
| set() -> new empty set object
| set(iterable) -> new set object
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| Build an unordered collection of unique elements.
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| Methods defined here:
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| __and__(...)
| x.__and__(y) <==> x&y
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| __cmp__(...)
| x.__cmp__(y) <==> cmp(x,y)
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| __contains__(...)
| x.__contains__(y) <==> y in x.
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| __eq__(...)
| x.__eq__(y) <==> x==y
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| __ge__(...)
| x.__ge__(y) <==> x>=y
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| __getattribute__(...)
| x.__getattribute__('name') <==> x.name
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| __gt__(...)
| x.__gt__(y) <==> x>y
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| __iand__(...)
| x.__iand__(y) <==> x&y
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| __init__(...)
| x.__init__(...) initializes x; see help(type(x)) for signature
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| __ior__(...)
| x.__ior__(y) <==> x|y
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| __isub__(...)
| x.__isub__(y) <==> x-y
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| __iter__(...)
| x.__iter__() <==> iter(x)
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| __ixor__(...)
| x.__ixor__(y) <==> x^y
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| __le__(...)
| x.__le__(y) <==> x<=y
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| __len__(...)
| x.__len__() <==> len(x)
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| __lt__(...)
| x.__lt__(y) <==> x<y
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| __ne__(...)
| x.__ne__(y) <==> x!=y
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| __or__(...)
| x.__or__(y) <==> x|y
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| __rand__(...)
| x.__rand__(y) <==> y&x
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| __reduce__(...)
| Return state information for pickling.
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| __repr__(...)
| x.__repr__() <==> repr(x)
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| __ror__(...)
| x.__ror__(y) <==> y|x
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| __rsub__(...)
| x.__rsub__(y) <==> y-x
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| __rxor__(...)
| x.__rxor__(y) <==> y^x
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| __sizeof__(...)
| S.__sizeof__() -> size of S in memory, in bytes
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| __sub__(...)
| x.__sub__(y) <==> x-y
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| __xor__(...)
| x.__xor__(y) <==> x^y
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| add(...)
| Add an element to a set.
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| This has no effect if the element is already present.
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| clear(...)
| Remove all elements from this set.
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| copy(...)
| Return a shallow copy of a set.
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| difference(...)
| Return the difference of two or more sets as a new set.
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| (i.e. all elements that are in this set but not the others.)
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| difference_update(...)
| Remove all elements of another set from this set.
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| discard(...)
| Remove an element from a set if it is a member.
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| If the element is not a member, do nothing.
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| intersection(...)
| Return the intersection of two or more sets as a new set.
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| (i.e. elements that are common to all of the sets.)
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| intersection_update(...)
| Update a set with the intersection of itself and another.
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| isdisjoint(...)
| Return True if two sets have a null intersection.
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| issubset(...)
| Report whether another set contains this set.
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| issuperset(...)
| Report whether this set contains another set.
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| pop(...)
| Remove and return an arbitrary set element.
| Raises KeyError if the set is empty.
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| remove(...)
| Remove an element from a set; it must be a member.
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| If the element is not a member, raise a KeyError.
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| symmetric_difference(...)
| Return the symmetric difference of two sets as a new set.
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| (i.e. all elements that are in exactly one of the sets.)
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| symmetric_difference_update(...)
| Update a set with the symmetric difference of itself and another.
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| union(...)
| Return the union of sets as a new set.
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| (i.e. all elements that are in either set.)
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| update(...)
