Pthon魔术方法（Magic Methods）-hash

　　　　　　　　　　　　　　Pthon魔术方法（Magic Methods）-hash

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　作者：尹正杰

版权声明：原创作品，谢绝转载！否则将追究法律责任。

一.hash方法

__hash__:
　　内建函数hash()调用的返回值，返回一个整数。如果定义这个方法该类的实例就可hash。

__eq__:
　　对应"=="操作符，判断两个对象内容是否相等，返回bool值。
　　定义了这个方法，如果不提供"__hash__"方法，那么实例将不可hash了。
　　"__hash__"方法只是返回一个hash值作为set的key，但是去重还需要"__eq__"来判断两个对象是否相等。
　　hash值相同，只是hash冲突，不能说明两个对象是相等的。因此一半来说提供"__hash__"方法是为了作为set或者dict的key，如果去重同时要提供"__eq__"方法。
　
温馨提示：
　　不可hash对象isinstance(p1,collections.Hashable)一定为False。
　　去重需要提供"__eq__"方法。

二.案例展示

 #！/usr/bin/env python
 #_*_conding:utf-8_*_
 #@author :yinzhengjie
 #blog:http://www.cnblogs.com/yinzhengjie

 """
     设计二维坐标类Point，使其成为可hash类型，并比较2个坐标的实例是否相等。
 """

 from collections import Hashable

 class Point:
     def __init__(self,x,y):
         self.x = x
         self.y = y

     def __hash__(self):
         return hash((self.x,self.y))

     def __eq__(self, other):
         if self is other:
             return True
         else:
             return self.x == other.x and self.y == other.y

     def __repr__(self):
         return "{}:{}".format(self.x,self.y)

 p1 = Point(10,20)
 p2 = Point(10,20)

 print(hash(p1))
 print(hash(p2))

 print(p1 is p2)
 print(p1 == p2)
 print(hex(id(p1)),hex(id(p2)))
 print(set((p1,p2)))
 print(isinstance(p1,Hashable))

 #以上代码执行结果如下:
 3713074054217192181
 3713074054217192181
 False
 True
 0x137152b7888 0x137152b9348
 {10:20}
 True

三.小时牛刀

1>.list类实例为什么不可hash？

 class list(object):
     """
     Built-in mutable sequence.

     If no argument is given, the constructor creates a new empty list.
     The argument must be an iterable if specified.
     """
     def append(self, *args, **kwargs): # real signature unknown
         """ Append object to the end of the list. """
         pass

     def clear(self, *args, **kwargs): # real signature unknown
         """ Remove all items from list. """
         pass

     def copy(self, *args, **kwargs): # real signature unknown
         """ Return a shallow copy of the list. """
         pass

     def count(self, *args, **kwargs): # real signature unknown
         """ Return number of occurrences of value. """
         pass

     def extend(self, *args, **kwargs): # real signature unknown
         """ Extend list by appending elements from the iterable. """
         pass

     def index(self, *args, **kwargs): # real signature unknown
         """
         Return first index of value.

         Raises ValueError if the value is not present.
         """
         pass

     def insert(self, *args, **kwargs): # real signature unknown
         """ Insert object before index. """
         pass

     def pop(self, *args, **kwargs): # real signature unknown
         """
         Remove and return item at index (default last).

         Raises IndexError if list is empty or index is out of range.
         """
         pass

     def remove(self, *args, **kwargs): # real signature unknown
         """
         Remove first occurrence of value.

         Raises ValueError if the value is not present.
         """
         pass

     def reverse(self, *args, **kwargs): # real signature unknown
         """ Reverse *IN PLACE*. """
         pass

     def sort(self, *args, **kwargs): # real signature unknown
         """ Stable sort *IN PLACE*. """
         pass

     def __add__(self, *args, **kwargs): # real signature unknown
         """ Return self+value. """
         pass

     def __contains__(self, *args, **kwargs): # real signature unknown
         """ Return key in self. """
         pass

     def __delitem__(self, *args, **kwargs): # real signature unknown
         """ Delete self[key]. """
         pass

     def __eq__(self, *args, **kwargs): # real signature unknown
         """ Return self==value. """
         pass

     def __getattribute__(self, *args, **kwargs): # real signature unknown
         """ Return getattr(self, name). """
         pass

     def __getitem__(self, y): # real signature unknown; restored from __doc__
         """ x.__getitem__(y) <==> x[y] """
         pass

     def __ge__(self, *args, **kwargs): # real signature unknown
         """ Return self>=value. """
         pass

     def __gt__(self, *args, **kwargs): # real signature unknown
         """ Return self>value. """
         pass

     def __iadd__(self, *args, **kwargs): # real signature unknown
         """ Implement self+=value. """
         pass

     def __imul__(self, *args, **kwargs): # real signature unknown
         """ Implement self*=value. """
         pass

     def __init__(self, seq=()): # known special case of list.__init__
         """
         Built-in mutable sequence.

         If no argument is given, the constructor creates a new empty list.
         The argument must be an iterable if specified.
         # (copied from class doc)
         """
         pass

     def __iter__(self, *args, **kwargs): # real signature unknown
         """ Implement iter(self). """
         pass

     def __len__(self, *args, **kwargs): # real signature unknown
         """ Return len(self). """
         pass

     def __le__(self, *args, **kwargs): # real signature unknown
         """ Return self<=value. """
         pass

     def __lt__(self, *args, **kwargs): # real signature unknown
         """ Return self<value. """
         pass

     def __mul__(self, *args, **kwargs): # real signature unknown
         """ Return self*value. """
         pass

     @staticmethod # known case of __new__
     def __new__(*args, **kwargs): # real signature unknown
         """ Create and return a new object.  See help(type) for accurate signature. """
         pass

     def __ne__(self, *args, **kwargs): # real signature unknown
         """ Return self!=value. """
         pass

     def __repr__(self, *args, **kwargs): # real signature unknown
         """ Return repr(self). """
         pass

     def __reversed__(self, *args, **kwargs): # real signature unknown
         """ Return a reverse iterator over the list. """
         pass

     def __rmul__(self, *args, **kwargs): # real signature unknown
         """ Return value*self. """
         pass

     def __setitem__(self, *args, **kwargs): # real signature unknown
         """ Set self[key] to value. """
         pass

     def __sizeof__(self, *args, **kwargs): # real signature unknown
         """ Return the size of the list in memory, in bytes. """
         pass

     __hash__ = None

class list(object):（源码解析）

2>.functools.lru_cache使用到的functools.HashedSeq类继承自list，为什么可hash？

 class _HashedSeq(list):
     """ This class guarantees that hash() will be called no more than once
         per element.  This is important because the lru_cache() will hash
         the key multiple times on a cache miss.

     """

     __slots__ = 'hashvalue'

     def __init__(self, tup, hash=hash):
         self[:] = tup
         self.hashvalue = hash(tup)

     def __hash__(self):
         return self.hashvalue

class _HashedSeq(list):源码解析