WIP: Add python+coverage example

Currently WIP because:

- `$(location)` and `$(rootpath)` make vars aren't working correctly
  - This is ultimately what we tried to fix with the
  - `PYTHON_COVERAGE_TARGET` variable, which was ultimately not
    accepted. See also next point.
- Does not use the new toolchain feature that supports adding
  coveragepy directly to the python toolchain
  - Added together with the new coveragepy support in
    bazelbuild/bazel#15590, and is probably better to use than the
    `$(rootpath)` and `pip_install` solution we're showing here
  - While I'd love to rewrite the blog post to show this instead, I'd
    need a bit more time for that
- Python imports are wonky
  - For some reason, the sibling module can only be imported under a
    parent `python` module. I confirmed this by looking through
    `PYTHON_PATH`
  - Seems to be a Bazel regression? Strangely nobody is complaining
    about this yet, we should probably raise an issue
- Not tested in remote execution
  - No access to an engflow setup to test this right this instant
  - Since we're not using a toolchain (and even if we were we're using
    `pip_install`), this requires a pip on the host executing the
    WORKSPACE, so there's a decent chance it won't work
- Requires bumping Bazel to a version with bazelbuild/bazel#15590
  merged, which currently is only available in prereleases

.bazelversion

@@ -1 +1 @@
-5.2.0
+6.0.0-pre.20221020.1

WORKSPACE

@@ -4,6 +4,24 @@ load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_file")
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
 
+http_archive(
+    name = "rules_python",
+    sha256 = "8c8fe44ef0a9afc256d1e75ad5f448bb59b81aba149b8958f02f7b3a98f5d9b4",
+    strip_prefix = "rules_python-0.13.0",
+    url = "https://github.com/bazelbuild/rules_python/archive/refs/tags/0.13.0.tar.gz",
+)
+
+load("@rules_python//python:pip.bzl", "pip_parse")
+
+pip_parse(
+    name = "coveragepy",
+    requirements = "//python:requirements.txt",
+)
+
+load("@coveragepy//:requirements.bzl", "install_deps")
+
+install_deps()
+
 # Some file dependencies
 http_file(
     name = "emacs",
@@ -167,17 +185,3 @@ load("@io_bazel_rules_go//go:deps.bzl", "go_register_toolchains", "go_rules_depe
 go_rules_dependencies()
 
 go_register_toolchains(version = "1.19.3")
-
-http_archive(
-    name = "io_bazel_rules_kotlin",
-    sha256 = "a57591404423a52bd6b18ebba7979e8cd2243534736c5c94d35c89718ea38f94",
-    urls = ["https://github.com/bazelbuild/rules_kotlin/releases/download/v1.6.0/rules_kotlin_release.tgz"],
-)
-
-load("@io_bazel_rules_kotlin//kotlin:repositories.bzl", "kotlin_repositories")
-
-kotlin_repositories()
-
-load("@io_bazel_rules_kotlin//kotlin:core.bzl", "kt_register_toolchains")
-
-kt_register_toolchains()

python/BUILD

@@ -0,0 +1,40 @@
+"""Example of a python build/test with coverage.
+"""
+
+load("@coveragepy//:requirements.bzl", "entry_point")
+
+alias(
+    name = "python_coverage_tools",
+    actual = entry_point("coverage"),
+)
+
+py_library(
+    name = "example",
+    srcs = [
+        "__init__.py",
+        "example.py",
+    ],
+)
+
+py_test(
+    name = "test_example",
+    size = "small",
+    srcs = ["test_example.py"],
+    env = {
+        # In theory this should be `$(execpath :python_coverage_tool)`,
+        #
+        # however this will not resolve correctly as Bazel will
+        # resolve this relative to the runfiles directory. Instead, we
+        # just manually specify the path in the runfiles directory the
+        # coverage tools will be placed in.
+        #
+        # For a better solution involving setting the coverage tool in
+        # the toolchain, see
+        # https://github.com/bazelbuild/bazel/pull/15590
+        "PYTHON_COVERAGE": "coveragepy_coverage/rules_python_wheel_entry_point_coverage",
+    },
+    deps = [
+        ":example",
+        ":python_coverage_tools",
+    ],
+)

python/example.py

@@ -0,0 +1,10 @@
+def fizzbuzz(i: int) -> str:
+    if i % 3 == 0:
+        if i % 5 == 0:
+            return "FizzBuzz"
+        else:
+            return "Fizz"
+    elif i % 5 == 0:
+        return "Buzz"
+    else:
+        return str(i)

python/requirements.txt

@@ -0,0 +1 @@
+coverage==6.5.0

python/test_example.py

@@ -0,0 +1,19 @@
+import unittest
+
+# Bazel seems to be setting the PYTHON_PATH wrong, resulting in a
+# parent "python" module. This might be a regression.
+from python.example import fizzbuzz
+
+
+class ExampleTest(unittest.TestCase):
+    def test_fizzbuzz(self):
+        self.assertEqual(fizzbuzz(1), "1")
+        self.assertEqual(fizzbuzz(2), "2")
+        self.assertEqual(fizzbuzz(3), "Fizz")
+        self.assertEqual(fizzbuzz(4), "4")
+        self.assertEqual(fizzbuzz(5), "Buzz")
+        self.assertEqual(fizzbuzz(15), "FizzBuzz")
+
+
+if __name__ == "__main__":
+    unittest.main()