python set.__contains__ 的意外行为

2022-01-17 00:00:00 python list set

问题描述

从 __contains__ 文档中借用文档

print set.__contains__.__doc__
x.__contains__(y) <==> y in x.

这似乎适用于 int、basestring 等原始对象.但对于定义 __ne__ 和 __eq__ 方法的用户定义对象，我感到意外行为.这是一个示例代码:

This seems to work fine for primitive objects such as int, basestring, etc. But for user-defined objects that define the __ne__ and __eq__ methods, I get unexpected behavior. Here is a sample code:

class CA(object):
  def __init__(self,name):
    self.name = name

  def __eq__(self,other):
    if self.name == other.name:
      return True
    return False

  def __ne__(self,other):
    return not self.__eq__(other)

obj1 = CA('hello')
obj2 = CA('hello')

theList = [obj1,]
theSet = set(theList)

# Test 1: list
print (obj2 in theList)  # return True

# Test 2: set weird
print (obj2 in theSet)  # return False  unexpected

# Test 3: iterating over the set
found = False
for x in theSet:
  if x == obj2:
    found = True

print found   # return True

# Test 4: Typcasting the set to a list
print (obj2 in list(theSet))  # return True

那么这是一个错误还是一个功能?

So is this a bug or a feature?

解决方案

对于sets和dicts，需要定义__hash__.任何两个相等的对象都应该散列相同，以便在 sets 和 dicts 中获得一致/预期的行为.

For sets and dicts, you need to define __hash__. Any two objects that are equal should hash the same in order to get consistent / expected behavior in sets and dicts.

我建议使用 _key 方法，然后在需要比较项目部分的任何地方引用它，就像从 调用 __eq____ne__ 而不是重新实现它:

I would reccomend using a _key method, and then just referencing that anywhere you need the part of the item to compare, just as you call __eq__ from __ne__ instead of reimplementing it:

class CA(object):
  def __init__(self,name):
    self.name = name

  def _key(self):
    return type(self), self.name

  def __hash__(self):
    return hash(self._key())

  def __eq__(self,other):
    if self._key() == other._key():
      return True
    return False

  def __ne__(self,other):
    return not self.__eq__(other)

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