python 这个类如何在不实现“next”的情况下实现“__iter__”方法？
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How does this class implement the "__iter__" method without implementing "next"?
提问by zjm1126
I have the following code in django.template:
我在 django.template 中有以下代码：
class Template(object):
def __init__(self, template_string, origin=None, name='<Unknown Template>'):
try:
template_string = smart_unicode(template_string)
except UnicodeDecodeError:
raise TemplateEncodingError("Templates can only be constructed from unicode or UTF-8 strings.")
if settings.TEMPLATE_DEBUG and origin is None:
origin = StringOrigin(template_string)
self.nodelist = compile_string(template_string, origin)
self.name = name
def __iter__(self):
for node in self.nodelist:
for subnode in node:
yield subnode
def render(self, context):
"Display stage -- can be called many times"
return self.nodelist.render(context)
The part I am confused about is below. How does this __iter__method work? I can't find any corresponding nextmethod.
我感到困惑的部分如下。这种__iter__方法是如何工作的？我找不到任何相应的next方法。
def __iter__(self):
for node in self.nodelist:
for subnode in node:
yield subnode
This is the only way that I know how to implement __iter__:
这是我知道如何实施的唯一方法__iter__：
class a(object):
def __init__(self,x=10):
self.x = x
def __iter__(self):
return self
def next(self):
if self.x > 0:
self.x-=1
return self.x
else:
raise StopIteration
ainst = a()
for item in aisnt:
print item
In your answers, please try to use code examples rather than text, because my English is not very good. Thank you.
在你的回答中，请尽量使用代码示例而不是文本，因为我的英语不是很好。谢谢你。
回答by Mark Peters
From the docs:
从文档：
If a container object's
__iter__()method is implemented as a generator, it will automatically return an iterator object (technically, a generator object) supplying the__iter__()and__next__()methods.
如果容器对象的
__iter__()方法被实现为生成器，它将自动返回一个迭代器对象（从技术上讲，一个生成器对象）提供__iter__()和__next__()方法。
Here is your provided example using a generator:
这是您提供的使用生成器的示例：
class A():
def __init__(self, x=10):
self.x = x
def __iter__(self):
for i in reversed(range(self.x)):
yield i
a = A()
for item in a:
print(item)
回答by catchmeifyoutry
That __iter__method returns a python generator(see the documentation), as it uses the yieldkeyword.
The generator will provide the next() method automatically; quoting the documentation:
该__iter__方法返回一个 python生成器（请参阅文档），因为它使用yield关键字。生成器会自动提供 next() 方法；引用文档：
What makes generators so compact is that the __iter__() and next() methods are created automatically.
生成器如此紧凑的原因是 __iter__() 和 next() 方法是自动创建的。
EDIT:
编辑：
Generators are really useful. If you are not familiar with them, I suggest you readup on them, and play around with some test code.
发电机真的很有用。如果您不熟悉它们，我建议您阅读它们，并尝试一些测试代码。
Here is some more info on iterators and generators from StackOverflow.
这里有更多关于StackOverflow迭代器和生成器的信息。
