python - 为什么 Python 中的类型返回 True？

Question

我的理解是否正确，if 语句只有在计算结果为 True 时才会“执行”？

如果是这样，类型返回True的目的是什么？

这条规则背后的逻辑是什么？

有人可以给我一个例子来说明它在哪里有用吗？

示例:

""" Example """

def test(x):
    if float:
        print("success")

test(9)
test('\ntesting')

返回:

Answer 1

来自文档:

Any object can be tested for truth value, for use in an if or while condition or as operand of the Boolean operations below. The following values are considered false:

None

False

zero of any numeric type, for example, 0, 0L, 0.0, 0j.

any empty sequence, for example, '', (), [].

any empty mapping, for example, {}.

instances of user-defined classes, if the class defines a __nonzero__() or __len__() method, when that method returns the integer zero or bool value False. [1]

> All other values are considered true — so objects of many types are always true.

Operations and built-in functions that have a Boolean result always return 0 or False for false and 1 or True for true, unless otherwise stated. (Important exception: the Boolean operations or and and always return one of their operands.)

由于所有类型都是从对象派生的，__nonzero__不返回 False 或整数零，并且它们没有 __len__方法，则视为 True。

还有一件有趣的事情: __nonzero__ float 为<slot wrapper '__nonzero__' of 'float' objects>

以下是一些相关信息: What is a wrapper_descriptor, and why is Foo.__init__() one in this case?