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

Question

I have written a custom container object.

According to this page, I need to implement this method on my object:

__iter__(self)

However, upon following up the link to Iterator Types in the Python reference manual, there are no examples given of how to implement your own.

Can someone post a snippet (or link to a resource), that shows how to do this?

The container I am writing, is a map (i.e. stores values by unique keys). dicts can be iterated like this:

for k, v in mydict.items()

In this case I need to be able to return two elements (a tuple?) in the iterator. It is still not clear how to implement such an iterator (despite the several answers that have been kindly provided). Could someone please shed some more light on how to implement an iterator for a map-like container object? (i.e. a custom class that acts like a dict)?

Answer 1

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