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?
Now suppose I have a class foo with attribute bar. How can I define its truth value, so that, say, it will be evaluated to True if bar % 2 == 0 and False otherwise?
例如:
myfoo = foo()
myfoo.bar = 3
def a(foo):
if foo:
print "spam"
else:
print "eggs"
所以，a(myfoo) 应该打印 eggs".
__nonzero__:请参阅 Python 2 文档以了解 __nonzero__.
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()
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