Fluent Python, 2nd Edition

Chapter 18. with, match, and else Blocks

Context managers may end up being almost as important as the subroutine itself. We’ve only scratched the surface with them. […] Basic has a with statement, there are with statements in lots of languages. But they don’t do the same thing, they all do something very shallow, they save you from repeated dotted [attribute] lookups, they don’t do setup and tear down. Just because it’s the same name don’t think it’s the same thing. The with statement is a very big deal.¹

Raymond Hettinger, eloquent Python evangelist

This chapter is about control flow features that are not so common in other languages, and for this reason tend to be overlooked or underused in Python. They are:

• The with statement and context manager protocol

• Pattern matching with match/case

• The else clause in for, while, and try statements

The with statement sets up a temporary context and reliably tears it down, under the control of a context manager object. This prevents errors and reduces boilerplate code, making APIs at the same time safer and easier to use. Python programmers are finding lots of uses for with blocks beyond automatic file closing.

We’ve seen pattern matching in previous chapters, but here we’ll see how the grammar of a language can be expressed as sequence patterns. That observation explains why match/case is an effective tool to create language processors that are easy to understand and extend. We’ll study a complete interpreter for a small but functional ...