pytest Quick Start Guide

How does pytest do it?

By default, Python's assert statement does not provide any details when it fails, but as we just saw, pytest shows a lot of information about the variables and expressions involved in a failed assertion. So how does pytest do it?

Pytest is able to provide useful exceptions because it implements a mechanism called assertion rewriting.

Assertion rewriting works by installing a custom import hook that intercepts the standard Python import mechanism. When pytest detects that a test file (or plugin) is about to be imported, instead of loading the module, it first compiles the source code into an abstract syntax tree (AST) using the built-in ast module. Then, it searches for any assert statements and rewrites them so that ...