Iterators

• Anything that can be done with generators can also be done with class-based iterators.
• What makes generators so compact is that the __iter__() and __next__() methods are created automatically.

Overview

• When we write for loop, it internally calls iter() on the container object.
• The iter() function returns an iterator object.
• The iterator object has a __next__() method that returns the next item in the container.
• The __next__() method is called repeatedly until it raises StopIteration exception, which tells the for loop to stop iterating.

>>> s = 'abc'
>>> it = iter(s)
>>> it
<str_iterator object at 0x10c90e650>
>>> next(it)
'a'
>>> next(it)
'b'
>>> next(it)
'c'
>>> next(it)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
    next(it)
StopIteration

---

Code

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

    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('abcdef')

for c in rev:
    print(c)