Small example of a PYTHONPATH issue with rules_python.
If you run bazel test //pw_transfer/py:foo_test, you'll get a test failure
that includes a printout of the PYTHONPATH. The PYTHONPATH look roughly like
this (with some directory prefixes removed for clarity):
~/src/rules_python_path/pw_transfer/py/tests
[prefix]/sandbox/linux-sandbox/1/execroot/rules_python_example/bazel-out/k8-fastbuild/bin/pw_transfer/py/foo_test.runfiles
[prefix]/sandbox/linux-sandbox/1/execroot/rules_python_example/bazel-out/k8-fastbuild/bin/pw_transfer/py/foo_test.runfiles/com_google_protobuf/python
[prefix]/sandbox/linux-sandbox/1/execroot/rules_python_example/bazel-out/k8-fastbuild/bin/pw_transfer/py/foo_test.runfiles/rules_python_example
[prefix]/sandbox/linux-sandbox/1/execroot/rules_python_example/bazel-out/k8-fastbuild/bin/pw_transfer/py/foo_test.runfiles/rules_python_example/pw_transfer/py
[prefix]/sandbox/linux-sandbox/1/execroot/rules_python_example/bazel-out/k8-fastbuild/bin/pw_transfer/py/foo_test.runfiles/com_google_protobuf
[prefix]/sandbox/linux-sandbox/1/execroot/rules_python_example/bazel-out/k8-fastbuild/bin/pw_transfer/py/foo_test.runfiles/python3_10_x86_64-unknown-linux-gnu
[prefix]/sandbox/linux-sandbox/1/execroot/rules_python_example/bazel-out/k8-fastbuild/bin/pw_transfer/py/foo_test.runfiles/six
[prefix]/execroot/rules_python_example/external/python3_10_x86_64-unknown-linux-gnu/lib/python310.zip
[prefix]/execroot/rules_python_example/external/python3_10_x86_64-unknown-linux-gnu/lib/python3.10
[prefix]/execroot/rules_python_example/external/python3_10_x86_64-unknown-linux-gnu/lib/python3.10/lib-dynload
~/.local/lib/python3.10/site-packages
[prefix]/execroot/rules_python_example/external/python3_10_x86_64-unknown-linux-gnu/lib/python3.10/site-packages
Notice that runfiles/com_google_protobuf/python and runfiles/rules_python_example appear before rules_python_example/pw_transfer/py. This is the cause of the ImportError in the test.
You can "fix" the error by removing the dependency on the py_proto_library
from //pw_transfer/py:foo. This is weird because py_proto_library is also a
dependency of //pw_transfer/py:foo_test itself, and you don't need to remove
that dependency!
Changes in the transitive deps reordering PYTHONPATH entries is a problem.