Using the for statement with iterable collections

Python allows us to use the for statement with any kind of collection. We can write a statement like for x in coll to process list, set, or the keys of a dict. This works because all of the Python collections have common abstract base classes, defined in the collections.abc module.

This works via a common feature of the base classes, Sequence, Set, and Mapping. The Iterable mix in the class is part of each class definition. The implementation of this abstraction is our guarantee that all of the built-in collections will cooperate with the for statement.

Let's open up the internals to see how it works. We'll use this compound for statement as a concrete example:

for x in coll:
    print(x)

Conceptually, this compound statement starts with something very much like this assignment: coll_i=iter(coll). This will get an iterator object for the coll collection. This iter() function will leverage the special method __iter__() to produce the...