Add coverage reporting to the test suite

- the nox session "test-coverage" triggers further nox sessions
  that run the test suite for all supported Python versions
- the nox session "_test-coverage-run" runs the test suite for
  a particular Python version using the coverage tool
- the nox session "_test-coverage-report" combines the individual
  coverage reports

.gitignore

@@ -1,4 +1,5 @@
 .cache/
+dist/
 poetry.toml
 **/__pycache__/
 .venv/

noxfile.py

@@ -2,6 +2,7 @@
 
 import collections
 import pathlib
+import random
 import re
 import tempfile
 from collections.abc import Mapping
@@ -145,14 +146,7 @@ def lint(session: nox.Session) -> None:
     session.run("ruff", "check", *locations)
 
 
-@nox_session(python=SUPPORTED_PYTHONS)
-def test(session: nox.Session) -> None:
-    """Test code with `pytest`."""
-    start(session)
-
-    install_unpinned(session, "-e", ".")  # "-e" makes session reuseable
-    install_pinned(
-        session,
+TEST_DEPENDENCIES = (
     "packaging",
     "pytest",
     "pytest-cov",
@@ -160,6 +154,15 @@ def test(session: nox.Session) -> None:
     "xdoctest",
 )
 
+
+@nox_session(python=SUPPORTED_PYTHONS)
+def test(session: nox.Session) -> None:
+    """Test code with `pytest`."""
+    start(session)
+
+    install_unpinned(session, "-e", ".")  # "-e" makes session reuseable
+    install_pinned(session, *TEST_DEPENDENCIES)
+
     args = session.posargs or (
         "--cov",
         "--no-cov-on-fail",
@@ -169,6 +172,58 @@ def test(session: nox.Session) -> None:
     session.run("pytest", *args)
 
 
+_magic_number = random.randint(0, 987654321)  # noqa: S311
+
+
+@nox_session(name="test-coverage", python=MAIN_PYTHON, reuse_venv=True)
+def test_coverage(session: nox.Session) -> None:
+    """Report the combined coverage statistics.
+
+    Run the test suite for all supported Python versions
+    and combine the coverage statistics.
+    """
+    install_pinned(session, "coverage")
+
+    session.run("python", "-m", "coverage", "erase")
+
+    for version in SUPPORTED_PYTHONS:
+        session.notify(f"_test-coverage-run-{version}", (_magic_number,))
+    session.notify("_test-coverage-report", (_magic_number,))
+
+
+@nox_session(name="_test-coverage-run", python=SUPPORTED_PYTHONS, reuse_venv=False)
+def test_coverage_run(session: nox.Session) -> None:
+    """Measure the test coverage."""
+    do_not_reuse(session)
+    do_not_run_directly(session)
+
+    start(session)
+
+    session.install(".")
+    install_pinned(session, "coverage", *TEST_DEPENDENCIES)
+
+    session.run(
+        "python",
+        "-m",
+        "coverage",
+        "run",
+        "-m",
+        "pytest",
+        TESTS_LOCATION,
+    )
+
+
+@nox_session(name="_test-coverage-report", python=MAIN_PYTHON, reuse_venv=True)
+def test_coverage_report(session: nox.Session) -> None:
+    """Report the combined coverage statistics."""
+    do_not_run_directly(session)
+
+    install_pinned(session, "coverage")
+
+    session.run("python", "-m", "coverage", "combine")
+    session.run("python", "-m", "coverage", "report", "--fail-under=100")
+
+
 @nox_session(name="test-docstrings", python=MAIN_PYTHON)
 def test_docstrings(session: nox.Session) -> None:
     """Test docstrings with `xdoctest`."""
@@ -179,6 +234,21 @@ def test_docstrings(session: nox.Session) -> None:
     session.run("xdoctest", "src/lalib")
 
 
+def do_not_reuse(session: nox.Session, *, raise_error: bool = True) -> None:
+    """Do not reuse a session with the "-r" flag."""
+    if session._runner.venv._reused:  # noqa:SLF001
+        if raise_error:
+            session.error('The session must be run without the "-r" flag')
+        else:
+            session.warn('The session must be run without the "-r" flag')
+
+
+def do_not_run_directly(session: nox.Session) -> None:
+    """Do not run a session with `nox -s SESSION_NAME` directly."""
+    if not session.posargs or session.posargs[0] != _magic_number:
+        session.error("This session must not be run directly")
+
+
 def start(session: nox.Session) -> None:
     """Show generic info about a session."""
     if session.posargs:

poetry.lock

@@ -1048,4 +1048,4 @@ tests-strict = ["pytest (==4.6.0)", "pytest (==6.2.5)", "pytest-cov (==3.0.0)"]
 [metadata]
 lock-version = "2.0"
 python-versions = "^3.9"
-content-hash = "3648b3426753e3e36e575d6b08fe8ab1b5f5443dcf2d1effaed5e921cdb40933"
+content-hash = "41aa1b0224786397f339cd01099c1a687cb57e16898426ef101fd66247d3bd5c"

pyproject.toml

@@ -60,6 +60,7 @@ pydoclint = { extras = ["flake8"], version = "^0.5" }
 ruff = "^0.6"
 
 # Test suite
+coverage = "^7.6"
 packaging = "^24.1"  # to test the version identifier
 pytest = "^8.3"
 pytest-cov = "^5.0"