How iteration works in Python
In Python, an iterator is an object that represents a stream of data. While iterators are available for containers, sequences in particular always support iteration.
Iterators have the __next__() method available (or the built-in next() function). Calling next() multiple times returns successive items from the data stream. When no more items are available, a StopIteration exception is thrown.
Any class can use an iterator by defining a container.__iter__() method. This method returns an iterator object, typically just self. This object is necessary to support the iterator protocol. Different types of iteration can be supported, with each one providing a specific iterator request. For example, a tree structure could support both breadth-first and depth-first traversals.
The iterator protocol mentioned previously actually comprises two methods: iterator.__next__() and iterator.__iter__(). (Notice that __iter__() has a different class compared to the one above.)
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