Writing generator functions

Many functions can be expressed neatly as generator expressions. Indeed, we've seen that almost any kind of mapping or filtering can be done as a generator expression. They can also be done with a built-in higher-order function, such as map() or filter(), or as a generator function. When considering multiple statement generator functions, we need to be cautious that we don't stray from the guiding principles of functional programming: stateless function evaluation.

Using Python for functional programming means walking on a knife edge between purely functional programming and imperative programming. We need to identify and isolate the places where we must resort to imperative Python code because there isn't a purely functional alternative available.

We're obligated to write generator functions when we need statement features of Python. Features, such as the following, aren't available in generator expressions:

• A with context to work with external resources. We'll...