在 Python 中，这个表达式“(a is b) == ( id(a) == id(b) )” 总是返回 True 吗？

Question

在 Python 中，表达式 (a is b) == ( id(a) == id(b) ) 似乎总是返回 True，其中 a 和 b 是引用某个对象的变量，因为 id 函数返回存储它们的内存，而 is 是用于对象标识。

有什么例外吗？

Answer 1

如果您仅使用is 与None 进行比较，我相信您应该这样做，那么这个问题是无关紧要的，因为None 有一个明确定义的地址

>>> id(None)
10306432
>>> id(None)
10306432
>>> a = None
>>> id(a)
10306432

我还没有遇到过a is b产生的结果与id(a) == id(b)不同的情况，但你还是不想随便使用is：

>>> a = 1
>>> b = 1
>>> id(a), id(b), id(1)
(10914496, 10914496, 10914496)

# 1 has a single address, is fails to recognize that a and b were created separately.

>>> a = 500
>>> b = 500
>>> id(a), id(b), id(500)
(140251806972464, 140251806973200, 140251806973744)

# Unlike the previous case, 500 is created multiple times, each with a different address

>>> a, b = 500, 500
>>> id(a), id(b), id(500)
(140251806972336, 140251806972336, 140251806972464)

# Several instances during the same initialization are created with a single address

>>> a, b = 500, 5*100
>>> id(a), id(b), id(500)
(140251806973104, 140251806973200, 140251806971280)

# However it only works if all are created in the same manner...

>>> a, b = 5*100, 5*100
>>> id(a), id(b), id(500)
(140251806971920, 140251806973392, 140251806973104)

# ... and they have to be created explicitely.


>>> a = 500 ; b = 500
>>> id(a), id(b), id(500)
(140251806973104, 140251806973104, 140251806972464)

# Separating with a semicolon does not change this fact...

>>> 500 is 500
True
>>> a is b
True
>>> a = 500
>>> a is 500
False
>>> a = 500 ; a is 500
True

# However being in the same line is not a basis for always having the same address, even if all values were created explicitly :

>>> any([a is 500 for a in [500]])
False

其他对象都有自己的古怪行为：

>>> a = 'hello'
>>> b = 'hello'
>>> id(a), id(b), id('hello')
(140251789045408, 140251789045408, 140251789045408)

>>> a = 'hello' + ' '
>>> b = 'hello' + ' '
>>> id(a), id(b), id('hello' + ' ')
(140251789044344, 140251789012472, 140251789012920)

>>> a = []
>>> b = []
>>> id(a), id(b), id([])
(140251789066632, 140251789069704, 140251789174216)

我会得出结论，is 和 id 的行为太不可靠，以至于 (a is b) == (id(a) == id(b)) 没有用处即使它是真的，至少在不可变对象的情况下是这样。

我个人绝不会想到在任何情况下都使用is

if x is None:
    pass

我只为可变对象或自定义类保留id。

Answer 2

(a is b) == ( id(a) == id(b) )

这个表达式总是True。 有两种可能的方式： 1.a和b都指向同一个对象

>>>a='Hello'
>>>b=a
>>> a is b
True
>>> id(a)==id(b)
True

1. “a”和“b”都引用不同的对象。

>>>a='hello'
>>>b='world'
>>>a is b
False
>>>id(a)==id(b)
False

id(a)==id(b) 做 a is b 所做的事情。

现在，Incase1 (a is b) == ( id(a) == id(b) ) 这是True==True，它返回True。 在第二种情况下，(a is b) == ( id(a) == id(b) ) 这是False==False，它返回True

来自Docs：

运算符is 和is not 测试对象的身份：x is y 当且仅当x 和y 为真> 是同一个对象。使用id() 函数确定对象的身份。 x 不是 y 产生反真值。