How to implement __iter__(self) for a container object (Python)

Question

I normally would use a generator function. Each time you use a yield statement, it will add an item to the sequence.

The following will create an iterator that yields five, and then every item in some_list.

def __iter__(self):
   yield 5
   yield from some_list

Pre-3.3, yield from didn't exist, so you would have to do:

def __iter__(self):
   yield 5
   for x in some_list:
      yield x

Another option is to inherit from the appropriate abstract base class from the `collections module as documented here.

In case the container is its own iterator, you can inherit from collections.Iterator. You only need to implement the next method then.

An example is:

>>> from collections import Iterator
>>> class MyContainer(Iterator):
...     def __init__(self, *data):
...         self.data = list(data)
...     def next(self):
...         if not self.data:
...             raise StopIteration
...         return self.data.pop()
...         
...     
... 
>>> c = MyContainer(1, "two", 3, 4.0)
>>> for i in c:
...     print i
...     
... 
4.0
3
two
1

While you are looking at the collections module, consider inheriting from Sequence, Mapping or another abstract base class if that is more appropriate. Here is an example for a Sequence subclass:

>>> from collections import Sequence
>>> class MyContainer(Sequence):
...     def __init__(self, *data):
...         self.data = list(data)
...     def __getitem__(self, index):
...         return self.data[index]
...     def __len__(self):
...         return len(self.data)
...         
...     
... 
>>> c = MyContainer(1, "two", 3, 4.0)
>>> for i in c:
...     print i
...     
... 
1
two
3
4.0

NB: Thanks to Glenn Maynard for drawing my attention to the need to clarify the difference between iterators on the one hand and containers that are iterables rather than iterators on the other.

usually __iter__() just return self if you have already define the next() method (generator object):

here is a Dummy example of a generator :

class Test(object):

    def __init__(self, data):
       self.data = data

    def next(self):
        if not self.data:
           raise StopIteration
        return self.data.pop()

    def __iter__(self):
        return self

but __iter__() can also be used like this: http://mail.python.org/pipermail/tutor/2006-January/044455.html

The "iterable interface" in python consists of two methods __next__() and __iter__(). The __next__ function is the most important, as it defines the iterator behavior - that is, the function determines what value should be returned next. The __iter__() method is used to reset the starting point of the iteration. Often, you will find that __iter__() can just return self when __init__() is used to set the starting point.

See the following code for defining a Class Reverse which implements the "iterable interface" and defines an iterator over any instance from any sequence class. The __next__() method starts at the end of the sequence and returns values in reverse order of the sequence. Note that instances from a class implementing the "sequence interface" must define a __len__() and a __getitem__() method.

class Reverse:
    """Iterator for looping over a sequence backwards."""
    def __init__(self, seq):
        self.data = seq
        self.index = len(seq)

    def __iter__(self):
        return self

    def __next__(self):
        if self.index == 0:
            raise StopIteration
        self.index = self.index - 1
        return self.data[self.index]

>>> rev = Reverse('spam')
>>> next(rev)   # note no need to call iter()
'm'
>>> nums = Reverse(range(1,10))
>>> next(nums)
9

If your object contains a set of data you want to bind your object's iter to, you can cheat and do this:

>>> class foo:
    def __init__(self, *params):
           self.data = params
    def __iter__(self):
        if hasattr(self.data[0], "__iter__"):
            return self.data[0].__iter__()
        return self.data.__iter__()
>>> d=foo(6,7,3,8, "ads", 6)
>>> for i in d:
    print i
6
7
3
8
ads
6