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 the variables used in the expression are kept so that they can be used to show more helpful messages if the assertion fails. Finally, it saves the rewritten pyc file to disk for caching.

This all might seem very magical, but the process is actually simple, deterministic, and, best of all, completely transparent.