Writing a custom iterator

Now we have all the tools to appreciate how we can write our own custom iterator. Let's first define what is an iterable and an iterator:

  • Iterable: An object is said to be iterable if it's capable of returning its members one at a time. Lists, tuples, strings, dicts, are all iterables. Custom objects that define either of __iter__ or __getitem__ methods are also iterables.
  • Iterator: An object is said to be an iterator if it represents a stream of data. A custom iterator is required to provide an implementation for __iter__ that returns the object itself, and an implementation for __next__, which returns the next item of the data stream until the stream is exhausted, at which point all successive calls to __next__ simply raise the StopIteration exception. Built-in functions such as iter and next are mapped to call __iter__ and __next__ on an object, behind the scenes.

Let's write an iterator that returns all the odd characters from a string first, and then the even ones.

iterators/iterator.py 

class OddEven:

    def __init__(self, data):
        self._data = data
        self.indexes = (list(range(0, len(data), 2)) +
            list(range(1, len(data), 2)))

    def __iter__(self):
        return self

    def __next__(self):
        if self.indexes:
            return self._data[self.indexes.pop(0)]
        raise StopIteration

oddeven = OddEven('ThIsIsCoOl!')
print(''.join(c for c in oddeven))  # TIICO!hssol

oddeven = OddEven('HoLa')  # or manually...
it = iter(oddeven)  # this calls oddeven.__iter__ internally
print(next(it))  # H
print(next(it))  # L
print(next(it))  # o
print(next(it))  # a

So, we needed to provide an implementation for __iter__ which returned the object itself, and then one for __next__. Let's go through it. What needs to happen is that we return _data[0], _data[2], _data[4], ..., _data[1], _data[3], _data[5], ... until we have returned every item in the data. In order to do this, we prepare a list, indexes, like [0, 2, 4, 6, ..., 1, 3, 5, ...], and while there is at least an element in it, we pop the first one and return the element from the data that is at that position, thereby achieving our goal. When indexes is empty, we raise StopIteration, as required by the iterator protocol.

There are other ways to achieve the same result, so go ahead and try to code a different one yourself. Make sure the end result works for all edge cases, empty sequences, sequences of length 1, 2, and so on.

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