Python - “if"中的逻辑评估顺序陈述 [英] Python - logical evaluation order in "if" statement

问题描述

在 Python 中我们可以这样做:

In Python we can do this:

if True or blah:
    print("it's ok") # will be executed

if blah or True: # will raise a NameError
    print("it's not ok")

class Blah:
    pass
blah = Blah()

if blah or blah.notexist:
    print("it's ok") # also will be executed

• 有人可以指点我有关此功能的文档吗?
• 是语言的实现细节还是特性?
• 利用此功能是一种好的编码风格吗?

推荐答案

or和and 短路，见布尔运算 文档:

表达式x and y首先计算x；如果 x 为 false，则返回其值；否则，评估 y 并返回结果值.

The expression x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned.

表达式x or y首先计算x；如果 x 为真，则返回其值；否则，评估 y 并返回结果值.

The expression x or y first evaluates x; if x is true, its value is returned; otherwise, y is evaluated and the resulting value is returned.

请注意，对于 和，如果 x 的计算结果为 True 值，y 仅 被计算.相反，对于 or，y 仅在 x 评估为 False 值时才评估.

Note how, for and, y is only evaluated if x evaluates to a True value. Inversely, for or, y is only evaluated if x evaluated to a False value.

对于第一个表达式True or blah，这意味着永远不会评估blah，因为第一部分已经是True.

For the first expression True or blah, this means that blah is never evaluated, since the first part is already True.

此外，您的自定义 Blah 类被认为是 True:

Furthermore, your custom Blah class is considered True:

在布尔运算的上下文中，以及当控制流语句使用表达式时，以下值被解释为假:False、None、数字零所有类型，以及空字符串和容器(包括字符串、元组、列表、字典、集合和frozensets).所有其他值都被解释为 true.(参见 __nonzero__() 特殊方法改变这一点.)

In the context of Boolean operations, and also when expressions are used by control flow statements, the following values are interpreted as false: False, None, numeric zero of all types, and empty strings and containers (including strings, tuples, lists, dictionaries, sets and frozensets). All other values are interpreted as true. (See the __nonzero__() special method for a way to change this.)

由于您的类没有实现 __nonzero__() 方法(也没有实现 __len__ 方法)，因此就布尔值而言，它被视为 True表达有关.

Since your class does not implement a __nonzero__() method (nor a __len__ method), it is considered True as far as boolean expressions are concerned.

在表达式 blah 或 blah.notexist 中，blah 因此为真，并且永远不会评估 blah.notexist.

In the expression blah or blah.notexist, blah is thus true, and blah.notexist is never evaluated.

经验丰富的开发人员经常有效地使用此功能，最常用于指定默认值:

This feature is used quite regularly and effectively by experienced developers, most often to specify defaults:

some_setting = user_supplied_value or 'default literal'
object_test = is_it_defined and is_it_defined.some_attribute

请小心链接这些内容，并在适用的情况下使用条件表达式.

Do be wary of chaining these and use a conditional expression instead where applicable.

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