Add coverage module for python (#4439)

The actual analysis is delegated to the well-known coverage.py package.

Author: jodersky

docs/modules/ROOT/pages/pythonlib/linting.adoc

@@ -17,3 +17,7 @@ include::partial$example/pythonlib/linting/1-ruff-format.adoc[]
 
 include::partial$example/pythonlib/linting/2-ruff-check.adoc[]
 
+== Code Coverage
+
+include::partial$example/pythonlib/linting/3-coverage.adoc[]
+

example/pythonlib/linting/3-coverage/build.mill

@@ -0,0 +1,44 @@
+// Mill's support for code coverage analysis is implemented by
+// https://coverage.readthedocs.io/[the coverage.py package].
+//
+// You can use it by extending `CoverageTests` in your test module.
+
+import mill._, pythonlib._
+
+object `package` extends RootModule with PythonModule {
+
+  object test extends PythonTests with TestModule.Pytest with CoverageTests
+
+}
+
+/** See Also: src/main.py */
+
+/** See Also: test/src/test_main.py */
+
+// You can generate a coverage report with the `coverageReport` task.
+
+/** Usage
+> mill test.coverageReport
+Name ...                        Stmts   Miss  Cover
+...------------------------------------------------
+.../src/main.py                 4      1    75%
+.../test/src/test_main.py       5      0   100%
+...------------------------------------------------
+TOTAL ...                       9      1    89%
+*/
+
+// The task also supports any arguments understood by the `coverage.py` module.
+// For example, you can use it to fail if a coverage threshold is not met:
+
+/** Usage
+> mill test.coverageReport --fail-under 90
+error: ...
+error: Coverage failure: total of 89 is less than fail-under=90
+*/
+
+// Other forms of reports can be generated:
+//
+// * `coverageHtml`
+// * `coverageJson`
+// * `coverageXml`
+// * `coverageLcov`

example/pythonlib/linting/3-coverage/src/main.py

@@ -0,0 +1,5 @@
+def f1():
+    pass
+
+def f2():
+    pass

example/pythonlib/linting/3-coverage/test/src/test_main.py

@@ -0,0 +1,8 @@
+import main
+
+def test_f1():
+    main.f1()
+
+def test_other():
+    assert True
+

pythonlib/src/mill/pythonlib/CoverageModule.scala

@@ -0,0 +1,133 @@
+package mill.pythonlib
+
+import mill._
+
+/**
+ * Code coverage via Python's [coverage](https://coverage.readthedocs.io/)
+ * package.
+ *
+ * ** Note that this is a helper trait, and you are unlikely to use this
+ * directly. If you're looking for including coverage across tests in your
+ * project, please use [[CoverageTests]] instead! **
+ *
+ * If you do want to use this module directly, please be aware that analyzing
+ * code coverage introduces "non-linear" changes to the execution task flow, and
+ * you will need to respect the following contract:
+ *
+ * 1. This trait defines a location where coverage data must be saved.
+ *
+ * 2. You need to define a `coverageTask` which is responsible for creating
+ *    coverage data in the before mentioned location. How this is done is up to
+ *    you. As an example, the [[CoverageTests]] module modifies `pythonOptions`
+ *    to prepend a `-m coverage` command line argument.
+ *
+ * 3. This trait defines methods that will a) invoke the coverage task b) assume
+ *    report data exists in the predefined location c) use that data to generate
+ *    coverage reports.
+ */
+trait CoverageModule extends PythonModule {
+
+  override def pythonToolDeps = Task {
+    super.pythonToolDeps() ++ Seq("coverage>=7.6.10")
+  }
+
+  /**
+   * The *location* (not the ref), where the coverage report lives. This
+   * intentionally does not return a PathRef, since it will be populated from
+   * other places.
+   */
+  def coverageDataFile: T[os.Path] = Task { T.dest / "coverage" }
+
+  /**
+   * The task to run to generate the coverage report.
+   *
+   * This task must generate a coverage report into the output directory of
+   * [[coverageDataFile]]. It is required that this file be readable as soon
+   * as this task returns.
+   */
+  def coverageTask: Task[_]
+
+  private case class CoverageReporter(
+      interp: os.Path,
+      env: Map[String, String]
+  ) {
+    def run(command: String, args: Seq[String]): Unit =
+      os.call(
+        (
+          interp,
+          "-m",
+          "coverage",
+          command,
+          args
+        ),
+        env = env,
+        stdout = os.Inherit
+      )
+  }
+
+  private def coverageReporter = Task.Anon {
+    CoverageReporter(
+      pythonExe().path,
+      Map(
+        "COVERAGE_FILE" -> coverageDataFile().toString
+      )
+    )
+  }
+
+  /**
+   * Generate a coverage report.
+   *
+   * This command accepts arguments understood by `coverage report`. For
+   * example, you can cause it to fail if a certain coverage threshold is not
+   * met: `mill coverageReport --fail-under 90`
+   */
+  def coverageReport(args: String*): Command[Unit] = Task.Command {
+    coverageTask()
+    coverageReporter().run("report", args)
+  }
+
+  /**
+   * Generate a HTML version of the coverage report.
+   */
+  def coverageHtml(args: String*): Command[Unit] = Task.Command {
+    coverageTask()
+    coverageReporter().run("html", args)
+  }
+
+  /**
+   * Generate a JSON version of the coverage report.
+   */
+  def coverageJson(args: String*): Command[Unit] = Task.Command {
+    coverageTask()
+    coverageReporter().run("json", args)
+  }
+
+  /**
+   * Generate an XML version of the coverage report.
+   */
+  def coverageXml(args: String*): Command[Unit] = Task.Command {
+    coverageTask()
+    coverageReporter().run("xml", args)
+  }
+
+  /**
+   * Generate an LCOV version of the coverage report.
+   */
+  def coverageLcov(args: String*): Command[Unit] = Task.Command {
+    coverageTask()
+    coverageReporter().run("lcov", args)
+  }
+
+}
+
+/** Analyze code coverage, starting from tests. */
+trait CoverageTests extends CoverageModule with TestModule {
+
+  override def pythonOptions = Task {
+    Seq("-m", "coverage", "run", "--data-file", coverageDataFile().toString) ++
+      super.pythonOptions()
+  }
+
+  override def coverageTask = testCached
+
+}