Question

我正在从collections.abc中覆盖MutableSet，我希望能够确定其实例何时等于True / False。

我知道进行比较的神奇方法，但是我正在寻找类似检查Python提供的空集/列表的行为。

class Example():
    pass

e = Example()

if e:
    print("This shall work - the default of an instance is True")

# What I'd like is something similar to...

if []:
    pass
else:
    print("This shall be false because it's empty, there wasn't a comparison")

我已经查看了食谱：Special methods Data model-Other various websites-我似乎找不到答案了：（

我最终希望能够去

class A:
    def __init__(self, value: int):
        self.value = value

    def __cool_equality_method__(self):
        return self.value > 5

a = A(10)
b = A(3)

if a:
    print("This happens")
if b:
    print("This doesn't happen")

Answer 1

__bool__简单吗？

class A:
    def __bool__(self):
        if not getattr(self, 'trueish', None):
            return False
        else:
            return True

a = A()
if a:
    print("Hello")
a.trueish = True
if a:
    print("a is True")

Answer 2

您需要在您的类上实现__bool__方法，该方法只是将您以前的__cool_equality_method__重命名为__bool__：

class A:
    def __init__(self, value: int):
        self.value = value

    def __bool__(self):
        return self.value > 5

a = A(10)
b = A(3)

if a:
    print("This happens")
if b:
    print("This doesn't happen")

"""
This happens
"""

Python-如何在实例为True或False时声明

2 个答案: