布尔身份== True vs True [英] Boolean identity == True vs is True

问题描述

使用if foo is None而不是if foo == None来测试值是否明确地是None是常规约定.

It is standard convention to use if foo is None rather than if foo == None to test if a value is specifically None.

如果要确定某个值是否恰好是True(而不仅仅是真值)，是否有任何理由使用if foo == True而不是if foo is True?这在CPython(2.x和3.x)，Jython，PyPy等实现之间是否有所不同?

If you want to determine whether a value is exactly True (not just a true-like value), is there any reason to use if foo == True rather than if foo is True? Does this vary between implementations such as CPython (2.x and 3.x), Jython, PyPy, etc.?

示例:例如说True用作要与值'bar'或任何其他真值相区别的单例值:

Example: say True is used as a singleton value that you want to differentiate from the value 'bar', or any other true-like value:

if foo is True: # vs foo == True
    ...
elif foo == 'bar':
    ...

是否存在使用if foo is True会产生与if foo == True不同的结果的情况?

Is there a case where using if foo is True would yield different results from if foo == True?

注意:我知道 Python布尔值-如果x :，如果x == True，则vs如果x为True .但是，它仅解决通常应使用if foo，if foo == True或if foo is True来确定foo是否具有真实值的情况.

NOTE: I am aware of Python booleans - if x:, vs if x == True, vs if x is True. However, it only addresses whether if foo, if foo == True, or if foo is True should generally be used to determine whether foo has a true-like value.

更新:根据 PEP 285 §规范:

值False和True将是单例，如None.

The values False and True will be singletons, like None.

推荐答案

如果要确定某个值是否完全为True(而不仅仅是类似True的值)，是否有任何理由使用foo == True而不是foo为True?

If you want to determine whether a value is exactly True (not just a true-like value), is there any reason to use if foo == True rather than if foo is True?

如果要确保foo确实是一个布尔值且值为True的布尔值，请使用is运算符.

If you want to make sure that foo really is a boolean and of value True, use the is operator.

否则，如果foo的类型实现了自己的__eq__()且与True相比返回了真值，则结果可能会出乎意料.

Otherwise, if the type of foo implements its own __eq__() that returns a true-ish value when comparing to True, you might end up with an unexpected result.

根据经验，应该始终将is与内置常量True，False和None一起使用.

As a rule of thumb, you should always use is with the built-in constants True, False and None.

这在CPython(2.x和3.x)，Jython，PyPy等实现之间是否有所不同?

Does this vary between implementations such as CPython (2.x and 3.x), Jython, PyPy, etc.?

从理论上讲，is将比==快，因为后者必须遵守类型的自定义__eq__实现，而is可以直接比较对象身份(例如内存地址).

In theory, is will be faster than == since the latter must honor types' custom __eq__ implementations, while is can directly compare object identities (e.g., memory addresses).

我并不是很了解各种Python实现的源代码，但是我认为其中大多数可以通过使用一些内部标志来优化魔术方法的存在，因此我怀疑您不会注意到练习中的速度差异.

I don't know the source code of the various Python implementations by heart, but I assume that most of them can optimize that by using some internal flags for the existence of magic methods, so I suspect that you won't notice the speed difference in practice.

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