Refactor running tests with nox

Instead of having a session that handles a large matrix of python
versions, use a session that leverages the default python. This reduces
the overhead of maintaining the `noxfile.py`.

The replacement session has an optional posarg `partial-coverage` which
will produce parallel-mode coverage files. This creates drop-in
replacement behavior for the GitHub action which runs the tests on its
own matrix of python versions and operating systems.

`coverage_combine` moves to a CI only session.

Author: Preocts

.github/workflows/python-tests.yml

@@ -15,7 +15,7 @@ on:
 
 jobs:
   run-tests-and-coverage:
-    name: "Run nox for tests and coverage"
+    name: "Run pytest with coverage."
     runs-on: "${{ matrix.os }}"
     strategy:
       fail-fast: false
@@ -40,14 +40,15 @@ jobs:
         with:
           python-version: "${{ matrix.python-version }}"
           allow-prereleases: true
+          cache: "pip"
 
       - name: "Install nox"
         run: |
-          python -m pip install --upgrade pip nox
+          python -m pip install --upgrade nox
 
       - name: "Run tests and coverage via nox"
         run: |
-          nox --session version_coverage-${{ matrix.python-version }}
+          nox --session test -- partial-coverage
 
       - name: "Save coverage artifact"
         uses: "actions/upload-artifact@6f51ac03b9356f520e9adb1b1b7802705f340c2b"
@@ -58,7 +59,7 @@ jobs:
           include-hidden-files: true
 
   coverage-compile:
-    name: "coverage compile"
+    name: "Compile coverage reports."
     needs: "run-tests-and-coverage"
     runs-on: "ubuntu-latest"
     steps:
@@ -69,10 +70,11 @@ jobs:
         uses: "actions/setup-python@0b93645e9fea7318ecaed2b359559ac225c90a2b"
         with:
           python-version: "3.12"
+          cache: "pip"
 
       - name: "Install nox"
         run: |
-          python -m pip install --upgrade pip nox
+          python -m pip install --upgrade nox
 
       - name: "Download coverage artifacts"
         uses: "actions/download-artifact@fa0a91b85d4f404e444e00e005971372dc801d16"

noxfile.py

@@ -4,6 +4,7 @@
 import pathlib
 import shutil
 import sys
+from functools import partial
 
 import nox
 
@@ -12,7 +13,6 @@
 TESTS_PATH = "tests"
 COVERAGE_FAIL_UNDER = 50
 DEFAULT_PYTHON = "3.12"
-PYTHON_MATRIX = ["3.9", "3.10", "3.11", "3.12", "3.13"]
 VENV_PATH = ".venv"
 REQUIREMENTS_PATH = "./requirements"
 
@@ -32,10 +32,9 @@
 ]
 
 # Define the default sessions run when `nox` is called on the CLI
-nox.options.default_venv_backend = "uv|virtualenv"
+nox.options.default_venv_backend = "virtualenv"
 nox.options.sessions = [
-    "version_coverage",
-    "coverage_combine",
+    "test",
     "mypy",
 ]
 
@@ -72,30 +71,37 @@ def dev(session: nox.Session) -> None:
         session.log(f"\n\nRun '{activate_command}' to enter the virtual environment.\n")
 
 
-@nox.session(python=PYTHON_MATRIX)
-def version_coverage(session: nox.Session) -> None:
-    """Run unit tests with coverage saved to partial file."""
+@nox.session(name="test")
+def run_tests_with_coverage(session: nox.Session) -> None:
+    """Run pytest with coverage, outputs console report and json."""
     print_standard_logs(session)
 
     session.install(".")
-    session.install("-r", "requirements/requirements.txt")
-    session.install("-r", "requirements/requirements-test.txt")
-    session.run("coverage", "run", "-p", "-m", "pytest", TESTS_PATH)
+    session.install("-r", f"{REQUIREMENTS_PATH}/requirements-test.txt")
 
+    coverage = partial(session.run, "python", "-m", "coverage")
 
-@nox.session(python=DEFAULT_PYTHON)
+    coverage("erase")
+
+    if "partial-coverage" in session.posargs:
+        coverage("run", "--parallel-mode", "--module", "pytest", TESTS_PATH)
+    else:
+        coverage("run", "--module", "pytest", TESTS_PATH)
+        coverage("report", "--show-missing", f"--fail-under={COVERAGE_FAIL_UNDER}")
+        coverage("json")
+
+
+@nox.session()
 def coverage_combine(session: nox.Session) -> None:
-    """Combine all coverage partial files and generate JSON report."""
+    """CI: Combine parallel-mode coverage files and produce reports."""
     print_standard_logs(session)
 
-    fail_under = f"--fail-under={COVERAGE_FAIL_UNDER}"
+    session.install("-r", f"{REQUIREMENTS_PATH}/requirements-test.txt")
 
-    session.install(".")
-    session.install("-r", "requirements/requirements.txt")
-    session.install("-r", "requirements/requirements-test.txt")
-    session.run("python", "-m", "coverage", "combine")
-    session.run("python", "-m", "coverage", "report", "-m", fail_under)
-    session.run("python", "-m", "coverage", "json")
+    coverage = partial(session.run, "python", "-m", "coverage")
+    coverage("combine")
+    coverage("report", "--show-missing", f"--fail-under={COVERAGE_FAIL_UNDER}")
+    coverage("json")
 
 
 @nox.session(python=DEFAULT_PYTHON)
@@ -109,14 +115,6 @@ def mypy(session: nox.Session) -> None:
     session.run("mypy", "-p", MODULE_NAME, "--no-incremental")
 
 
-@nox.session(python=False)
-def coverage(session: nox.Session) -> None:
-    """Generate a coverage report. Does not use a venv."""
-    session.run("coverage", "erase")
-    session.run("coverage", "run", "-m", "pytest", TESTS_PATH)
-    session.run("coverage", "report", "-m")
-
-
 @nox.session(python=DEFAULT_PYTHON)
 def build(session: nox.Session) -> None:
     """Build distribution files."""