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

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