将__iter__而不是类本身添加到元类中（假设Python 2.x）：
class Foo(object):
bar = "bar"
baz = 1
class __Metaclass__(type):
def __iter__(self):
for attr in dir(self):
if not attr.startswith("__"):
yield attr
对于Python 3.x，请使用
class MetaFoo(type):
def __iter__(self):
for attr in dir(self):
if not attr.startswith("__"):
yield attr
class Foo(Metaclass=MetaFoo):
bar = "bar"
baz = 1
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