how to unit test a macro I just want to ensure that for give Pants #general

how to unit-test a macro? I just want to ensure th...

elegant-art-7479

05/08/2025, 9:17 PM

how to unit-test a macro? I just want to ensure that for given inputs, it configures certain targets with specific values. I tried patching things like

files

docker_image

, but because no imports are allowed in the macro files, patching doesn't really work. I'd expect there are some utils that let us verify if given target functions were called

fast-nail-55400

05/09/2025, 2:15 AM

Maybe use an existing test for the pattern to use? https://github.com/pantsbuild/pants/blob/e82f3302fd2cfb9aa9fd3ac457397d94c65f747f/src/python/pants/backend/python/macros/python_requirements_test.py

fast-nail-55400

05/09/2025, 2:16 AM

You can use the macro in a BUILD file which exists only for the test. Then you can request the targets made by the macro using normal

RuleRunner

mechanisms and just check the fields on those targets.

fast-nail-55400

05/09/2025, 2:19 AM

(Although the linked file is for a "target generator" which is a different mechanism inside Pants rules. But the idea remains the same: use the macro with a

RuleRunner

and then query the engine for what it created.)

elegant-art-7479

05/09/2025, 1:56 PM

thanks for the hints! at first glance this approach seems to contain some accidental complexity since there's a need to use some extra mechanisms that I don't have to be aware as a user of just a single macro, but I'll try to dig deeper one day. It would be nice to have documented a minimal example of a unit test for a trivial macro, with all the moving parts

fast-nail-55400

05/09/2025, 2:16 PM

ah you want to test a macro which is in a regular code repository? I had thought it was to test a macro provided by a pants plugin. The mechanisms above are for testing Pants plugins.

elegant-art-7479

05/09/2025, 2:17 PM

> ah you want to test a macro which is in a regular code repository? yes, the macro is co-located with the code using it, and consumed via

Copy code

build_file_prelude_globs = [
    'pants_plugins/macros.py',
]

pants.toml

fast-nail-55400

05/09/2025, 2:19 PM

For a testing a macro in a regular code repository, here is a hacky solution: Write a regular test file. Load the code from the macro file into a string. Then just

eval

the string with appropriate test fixtures injected to stand in for the

BUILD

file symbols the macros uses. Check results.

elegant-art-7479

05/09/2025, 2:21 PM

oh ok,

eval

- I didn't think of it, and tried to actually import the macro from a test. Ok, I'll try it, thanks! I'm thinking about a feature request to make it less hacky, perhaps encapsulating what you suggest into some test utils

curved-manchester-66006

05/09/2025, 2:22 PM

👀 ooh, that would be a good trick

fast-nail-55400

05/09/2025, 2:22 PM

Copy code

with open("pants_plugins/macros.py") as f:
    macro = f.read()

generated_targets = []

def my_target_standin(*args, **kwargs):
    nonlocal generated_targets
    generated_targets.append((args, kwargs))

eval(macro, globals={"env": some_env_standin, "my_target": my_target_standin})

# check `generated targets`

fast-nail-55400

05/09/2025, 2:23 PM

^^ very rough sketch

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