When bazel meets pytest and rules_proto_grpc

Bazel is amazing in providing reproducible builds, but the rules_python is still under heavy development. Pytest usage from within bazel can be a bit tricky, as can be seen from:

1. A stack overflow answer where we make each test file a script and need to remember to add an if __name__ == "__main__" block.
2. Answer within the same thread where we create a wrapper macro, which allows us not need the solution from above.

All was well until I started to use rules_proto_grpc. I could not get the example code to work if I was using pytest as my testing framework.

Some important things to note:

1. rules_proto_grpc is defining a python library with usage of imports flag. This is needed because the path to the python files in the bazel-bin directory are bazel-bin/idl/path/to/build/file/name_pb/idl/for/import where idl/path/to/build/file segment mirrors the path of the BUILD file relative to your WORKSPACE and idl/for/import segment mirrors the import path found in the .proto file. Benefits of such approach as far as I can see are manyfold:
   1. The import path and the bazel path are decoupled.
   2. No two rules are going to overwrite the same files.
   3. Probably something else which I am not yet aware.
2. This imports usage in turn modifies the PYTHONPATH such that we could import the generated python package via from idl.for.import import foo_pb2 as foo_pb.
3. Everything works until pytest comes along, which is doing some test discovery and is also modifying the paths which python checks when importing idl.for.import. The way it is doing this is:
   1. We are running inside bazel-bin/lib/foo/foo_test.runfiles/<my-workspace-name> directory (assuming the test rule we are running from is called foo_test).
   2. Pytest crawls the directories it can see and notices that we have python files within idl/path/to/build/file/name_pb/idl/for/import and happily prepends the list of paths to search python modules from so our previously working idl/for/import stops working because both of the paths have idl in the prefix and Python checks the idl directory and does not go into idl/path/to/build/file/name_pb/idl directory because it is failing fast in this case.
   3. Everything works if pytest is doing the crawl after we load the idl.for.import module because of how python module caching works as alluded here.

So this repo shows a solution:

pass an extra --import-mode=importlib, which will become later the default.

If you know a better solution, let me know via an issue or a PR to this example!