Python 类型错误：“类型”对象不可迭代 - 迭代对象实例
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TypeError: 'type' object is not iterable - Iterating over object instances
提问by mirind4
I am working on a project and I would like to make one of my classes iterable. To the best of my knowledge I can do that with using metaclass.
我正在做一个项目，我想让我的一个类可迭代。据我所知，我可以使用元类来做到这一点。
First of all I would like to understand how metaclass works. Therefore I would like to present my own practicing example where I made a Car class. So here I would like to make my Car class objects iterable then I would like to print the names of them in the main function.
首先，我想了解元类是如何工作的。因此，我想展示我自己的练习示例，其中我制作了 Car 课程。所以在这里我想让我的 Car 类对象可迭代，然后我想在主函数中打印它们的名称。
The code example is the following:
代码示例如下：
__author__ = 'mirind4'
class IterableCar(type):
def __iter__(self):
return iter(self.__name__)
class Car(object):
__metaclass__ = IterableCar
def __init__(self, name):
self.name = name
if __name__=='__main__':
car1 = Car('Mercedes')
car2 = Car('Toyota')
for cars in Car:
print (cars.name)
But unfortunately I got an TypeError:
但不幸的是我得到了一个类型错误：
TypeError: 'type' object is not iterable
Would you be so kind as to tell me where I do the mistake in my code? So far I have checked similar problem-questions over this site and internet but I do not know what the problem is. I am using python 3.4. Thanks in advance!
你能不能告诉我我的代码哪里出错了？到目前为止，我已经通过这个网站和互联网检查了类似的问题，但我不知道问题是什么。我正在使用 python 3.4。提前致谢！
采纳答案by mgilson
As far as I can tell, making a class object iterable by using a metaclass works just fine:
据我所知，通过使用元类使类对象可迭代工作得很好：
from __future__ import print_function
class IterableCar(type):
def __iter__(cls):
return iter(cls.__name__)
class Car(object):
__metaclass__ = IterableCar
def __init__(self, name):
self.name = name
if __name__=='__main__':
car1 = Car('Mercedes')
car2 = Car('Toyota')
for cars in Car:
print (cars)
Results in:
结果是：
mgilson$ python ~/sandbox/test.py
C
a
r
Here's an example where I actually track the cars generated:
这是我实际跟踪生成的汽车的示例：
from __future__ import print_function
import weakref
class IterableCar(type):
_cars = weakref.WeakSet()
def __iter__(cls):
return iter(cls._cars)
def add_car(cls, car):
cls._cars.add(car)
class Car(object):
__metaclass__ = IterableCar
def __init__(self, name):
self.__class__.add_car(self)
self.name = name
if __name__=='__main__':
car1 = Car('Mercedes')
car2 = Car('Toyota')
for cars in Car:
print (cars.name)
Note that if you're using python3.x, to use a metaclass you do:
请注意，如果您使用的是 python3.x，要使用元类，请执行以下操作：
class Car(metaclass=IterableCar):
...
Rather than:
而不是：
class Car(object):
__metaclass__ = IterableCar
which likely explains the problem that you're experiencing.
这可能解释了您遇到的问题。
回答by chepner
To track instances of the class that are created, we'll start by adding a _carsattribute to each the class created by the metaclass. This will be set of weak references, so that the class itself does not prevent unused instances from being garbage-collected.
为了跟踪创建的类的实例，我们首先向_cars元类创建的每个类添加一个属性。这将是一组弱引用，因此类本身不会阻止未使用的实例被垃圾收集。
class IterableCar(type):
def __new__(meta, name, bases, attrs):
attrs['_cars'] = weaker.WeakSet()
return type.__new__(meta, name, bases, attrs)
To add the instances, we'll override __call__. Essentially, this is where you put code that you would ordinarily put in __new__or __init__when defining the class itself.
要添加实例，我们将覆盖__call__. 本质上，这是您放置通常放置的代码__new__或__init__在定义类本身时放置的位置。
def __call__(cls, *args, **kwargs):
rv = type.__call__(cls, *args, **kwargs)
cls._cars.add(rv)
return rv
And to make the class iterable by iterating over its set of instances,
并通过迭代其实例集来使类可迭代，
def __iter__(self):
return iter(self._cars)
Any class using IterableCarwill automatically track its instances.
任何使用的类IterableCar都会自动跟踪它的实例。
class Car(metaclass=IterableCar):
def __init__(self, name):
self.name = name
car1 = Car('Mercedes')
car2 = Car('Toyota')
for cars in Car:
print(cars.name)
