How does this class implement the "__iter__" method without implementing "next"?

Question

I have the following code in django.template:

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 next method.

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__:

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, try to use code examples rather than text, because my English is not very good.

Answer 1

From the docs:

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.

Here is your provided example using a generator:

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)

Answer 2

That __iter__method returns a python generator (see the documentation), as it uses the yield keyword. The generator will provide the next() method automatically; quoting the documentation:

What makes generators so compact is that the __iter__() and next() methods are created automatically.

EDIT:

Generators are really useful. If you are not familiar with them, I suggest you readup on them, and play around with some test code.

Here is some more info on iterators and generators from StackOverflow.