[Bug]: Module not found (in this case pytest)

[njlr]

What happened?

I have a complex project but I will try to capture the import parts.

requirements.txt contains pytest, among other things:

pytest==7.3.1

My Bazel target imports pytest:

load("@aspect_rules_py//py:defs.bzl", "py_binary", "py_pytest_main", "py_test")
load("@pypi//:requirements.bzl", "requirement")

# ...

py_test(
  name = "tests",
  srcs = [
    "tests/**/*.py",
  ],
  main = "test.py",
  python_version = "3.10.6",
  deps = [
    requirement("pytest"),
  ],
)

WORKSPACE parses the requirements:

load("@rules_python//python:pip.bzl", "pip_parse")

pip_parse(
  name = "pypi",
  requirements_lock = "//:requirements.txt",
  python_interpreter_target = python_3_10_6_interpreter,
)

load("@pypi//:requirements.bzl", pypi_install_deps = "install_deps")

pypi_install_deps()

By when I run the test, I get:

ModuleNotFoundError: No module named 'pytest'

Version

Development (host) and target OS/architectures:

Output of bazel --version: bazel 6.4.0

Version of the Aspect rules, or other relevant rules from your
WORKSPACE or MODULE.bazel file:

http_archive(
  name = "rules_python",
  sha256 = "d71d2c67e0bce986e1c5a7731b4693226867c45bfe0b7c5e0067228a536fc580",
  strip_prefix = "rules_python-0.29.0",
  url = "https://github.com/bazelbuild/rules_python/archive/refs/tags/0.29.0.tar.gz",
)

http_archive(
  name = "aspect_rules_py",
  sha256 = "04278ce23cc5c91a24b62ea00ac04c553fe40ca390943acf6684d367a681a871",
  strip_prefix = "rules_py-0.7.4",
  url = "https://github.com/aspect-build/rules_py/releases/download/v0.7.4/rules_py-v0.7.4.tar.gz",
)

Language(s) and/or frameworks involved: -

How to reproduce

See above

Any other information?

No response

[njlr]

It appears that the order of my toolchains was wrong.

This order works:

python_register_toolchains(
  name = "python3_10_6",
  python_version = "3.10.6",
  set_python_version_constraint = True,
)

load("@python3_10_6//:defs.bzl", python_3_10_6_interpreter = "interpreter")

# Default toolchain must go last
python_register_toolchains(
  name = "python3_9_16",
  python_version = "3.9.16",
)

load("@python3_9_16//:defs.bzl", python_interpreter = "interpreter")

Perhaps this needs a better error message?

[mattem]

That error message is from Python itself, so I'm not sure what rules_py could have done here. However, the toolchain ordering looks like it might be related to #363

[njlr]

It's generally quite hard to know which toolchain is being used and if you have configured things incorrectly.

From the usage, I was under the impression that 3.10.6 had been selected:

py_test(
  name = "tests",
  srcs = [
    "tests/**/*.py",
  ],
  main = "test.py",
  python_version = "3.10.6", # <----- Set here
  deps = [
    requirement("pytest"),
  ],
)

pip_parse(
  name = "pypi",
  requirements_lock = "//:requirements.txt",
  python_interpreter_target = python_3_10_6_interpreter, # <----- Matches here
)

But it actually depends on the order of toolchain registration.

If you declare a py_binary / py_test target that depends on a PIP package whose Python toolchain doesn't match, I think there should be an error or at least a warning.

[njlr]

Duplicate of #363