![Example iterable
In [8]: class B(object):
...: def next(self):
...: return 5
In [9]: class A(object):
...: def __iter__(self): // __iter__() is implemented
...: return B() // __iter__() also returns an iterator.](https://image.slidesharecdn.com/iteratorprotocol-171028120716/85/Iterator-protocol-6-320.jpg)
![StopIteration implementation
In [5]: class B(object):
...: def __init__(self):
...: self.i = 0
...: def next(self):
...: if self.i == 3:
...: raise StopIteration()
...: self.i += 1
...: return "boom"](https://image.slidesharecdn.com/iteratorprotocol-171028120716/85/Iterator-protocol-10-320.jpg)
Iterator protocol
- 1. Iterator Protocol Iterators and Iterables
- 2. Why iterator protocol? ● Writing memory optimized code. ● Understanding how Python web framework works. Eg: Django, Flask ● Writing simpler, abstracted code. We will see an example at the end.
- 3. Iterator Protocol Iterator Protocol needs two objects: ● Iterables ● Iterators
- 4. Iterables ● Any class which implements __iter__(). ● Must implement __iter__(). ● __iter__() **must** return an iterator.
- 5. Built in iter() ● Must be used with an iterable. ● Using with NotIterable() raises TypeError. ● Using with an Iterable() whose __iter__() doesn’t return iterator also raises TypeError.
- 6. Example iterable In [8]: class B(object): ...: def next(self): ...: return 5 In [9]: class A(object): ...: def __iter__(self): // __iter__() is implemented ...: return B() // __iter__() also returns an iterator.
- 7. Iterators ● Any class which implements next(). ● Must implement next().
- 8. Built in next() ● Must be used with an iterator. ● Using with NotIterator() raises TypeError.
- 9. StopIteration ● Used inside next() of an Iterator. ● Used to signal that the iterator has been fully consumed.
- 10. StopIteration implementation In [5]: class B(object): ...: def __init__(self): ...: self.i = 0 ...: def next(self): ...: if self.i == 3: ...: raise StopIteration() ...: self.i += 1 ...: return "boom"
- 11. Recap ● Iteraable must implement __iter__(). No need to implement next(). ● Iterator must implement next(). No need to implement __iter__() ● iter() must be used with Iterable. ● next() must be used with iterator.
- 12. How for loop works ● **for** works with an iterable which must have an underlying iterator. ● iter(iterable) is called which returns a iterator. ● next() of iterator is repeatedly called until StopIteration is raised from next().
- 13. How for loop works with Python lists ● A list is an iterable. It has __iter__. ● __iter__() of list returns an iterator. This iterator has next(). ● Try next(list_iterator) and check it raises StopIteration.
- 14. Real world usage of iterator protocol ● Implementing xrange(). Memory optimized code. ● Implementing Fibonacci numbers upto a maximum. ● Django fields and forms.