Writing a custom iterator

Now we have all the tools to appreciate how we can write our own custom iterator. Let's first define 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, and dictionaries are all iterables. Custom objects that define either of the __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 the __iter__() method that returns the object itself, and an implementation for the __next__() method that 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 the __iter__() and __next__() methods on an object, behind the scenes.

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