Exclude some test modules from assertion rewriting

[nicoddemus]

Sometimes users do some "black magic" like patching builtins (for example #2180) and that might completely break pytest.

While of course we should not encourage too much black magic, I was wondering if we could alleviate the problem for users which don't want or can't change their tests to disable assertion rewriting for some of the test modules.

I was thinking of something like the # encoding comment used by Python to declare the encoding of a file. An alternative would be to list some test files on pytest.ini, but I think the comment is better because it is written in the file itself, rather than far away from it.

How about # pytest-no-rewrite or something like this?

[The-Compiler]

I don't think this would help with #2180 though, would it? I suppose we'd still need to use open() to read the comment 😉

[nicoddemus]

@The-Compiler you're right. 😁

[nicoddemus]

From #743, there's an undocumented PYTEST_DONT_REWRITE in place already, perhaps we are halfway there already (save builtin open(), skip bytecode rewriting, docs, etc).

[jacebrowning]

PYTEST_DONT_REWRITE is exactly what I needed. Any chance we could get this formally documented?

My use case is a pytest plugin that manages its own assertion messages: https://github.com/jacebrowning/pytest-expecter

[nicoddemus]

Any chance we could get this formally documented?

FWIW this is now on the reference page. 😉

[laike9m]

Would be cool if there's a flag to always disable rewrite.

[blueyed]

@laike9m
--rewrite=plain?
You can use this in a config via addopts.

[laike9m]

Ah interesting. Found that in the documentation. Thanks!

https://docs.pytest.org/en/latest/assert.html#disabling-assert-rewriting