What makes an iterator an iterator?

[Terry Reedy]

An iterator is an object with a .__iter__  method that returns self and a 
.next method that either returns an object or raises StopIteration.

One very good way to get an iterator from an iterable is for .__iter__ to 
be a generator function.  When called, it returns an generator with 
.__iter__ and .next methods that work as specified.

words = "Norwegian Blues have beautiful plumage!".split()
print words

prints
['Norwegian', 'Blues', 'have', 'beautiful', 'plumage!']

class Parrot(object):
    def __init__(self, words):
        self.words = words
    def __iter__(self):
        for word in words:
            yield word

for word in Parrot(words):
    print word

prints

Norwegian
Blues
have
beautiful
plumage!

One can also make an iterable an (self) iterator by correctly writing the 
two needed methods.

class Parrot2(object):
    def __init__(self, words):
        self.words = words
    def __iter__(self):
        self.it = -1
        return self
    def next(self):
        self.it += 1
        try:
            return self.words[self.it]
        except IndexError:
            raise StopIteration

for word in Parrot2(words):
    print word

which prints the same 5 lines.  However, when an object is its own 
iterator, it can only be one iterator at a time (here, pointing at one 
place in the word list), whereas the first method allows multiple iterators 
(here, possibly pointing to different places in the list.

| Can you explain some of the details of why this code fails:

[snip all examples of bad code that violate the iterator rule by improperly 
writing .next as a generator function]

I think most people should learn how to write iterators correctly, as I 
gave examples of above, rather than worry about the details of how and why 
mis-written code fails with a particular version of a particular 
implementation.  So I won't go down that road.

Terry Jan Reedy