Python中对象的布尔值

Boolean value of objects in Python(Python中对象的布尔值)

问题描述

众所周知，Python 对对象有布尔值:如果一个类有一个 __len__ 方法，那么 __len__() 碰巧返回 0 的每个实例都会被评估为布尔值 False(例如，空列表).

As we know, Python has boolean values for objects: If a class has a __len__ method, every instance of it for which __len__() happens to return 0 will be evaluated as a boolean False (for example, the empty list).

事实上，如果每个可迭代的空自定义对象出现在布尔表达式中，它都会被评估为 False.

In fact, every iterable, empty custom object is evaluated as False if it appears in boolean expression.

现在假设我有一个带有属性bar 的类foo.我如何定义它的真值，例如，如果 bar % 2 == 0 和 False 否则它将被评估为 True?

myfoo = foo()
myfoo.bar = 3
def a(foo):
    if foo:
        print "spam"
    else:
        print "eggs"

class foo(object):
    def __nonzero__( self) :
        return self.bar % 2 == 0

def a(foo):
    if foo:
        print "spam"
    else:
        print "eggs"

def main():
    myfoo = foo()
    myfoo.bar = 3
    a(myfoo)

if __name__ == "__main__":
    main()