布尔标识 == True vs 是 True

问题描述

标准约定是使用 if foo is None 而不是 if foo == None 来测试一个值是否具体为 None.

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

如果您想确定一个值是否完全是 True (不仅仅是一个类似 true 的值)，是否有任何理由使用 if foo == True 而不是比if foo 为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:说 True 用作您想要与值 'bar' 或任何其他类似 true 的值区分开来的单例值:

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 - if x:, vs if x == True, vs if x is True.但是，它只处理 if foo、if foo == True 或 if foo is True 通常是否应该用于确定 foo 有一个类似于 true 的值.

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:

UPDATE: According to PEP 285 § Specification:

值 False 和 True 将是单例，例如 None.

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

推荐答案

如果您想确定一个值是否完全为 True(而不仅仅是类似 true 的值)，是否有任何理由使用 if foo == True 而不是 if foo is 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?

如果您想确保 foo 确实是一个布尔值且值为 True，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 的类型实现了它自己的 __eq__() 在与 True 比较时返回一个真值，你可能会导致意想不到的结果.

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，您应该始终将 is 与内置常量 True、False 和 None<一起使用/代码>.