ci: Enable static type checking with mypy (meltano#6707)

* ci: Enable static type checking with mypy

* List modules in meltano.core individually

.github/workflows/test.yml

@@ -255,3 +255,46 @@ jobs:
 
     - name: Upload coverage report
       uses: codecov/codecov-action@v3.1.0
+
+  mypy:
+    name: "Static type checking"
+    runs-on: ubuntu-latest
+    env:
+      FORCE_COLOR: 1
+
+    steps:
+    - name: Check out the repository
+      uses: actions/checkout@v3.0.2
+
+    - name: Setup Python 3.9
+      uses: actions/setup-python@v4.2.0
+      with:
+        python-version: '3.9'
+        architecture: x64
+        cache: 'pip'
+        cache-dependency-path: 'poetry.lock'
+
+    - name: Install Poetry
+      env:
+        PIP_CONSTRAINT: .github/workflows/resources/constraints.txt
+      run: |
+        pipx install poetry
+        poetry --version
+
+    - name: Upgrade pip
+      env:
+        PIP_CONSTRAINT: .github/workflows/resources/constraints.txt
+      run: |
+        pip install pip
+        pip --version
+
+    - name: Install Nox
+      env:
+        PIP_CONSTRAINT: .github/workflows/resources/constraints.txt
+      run: |
+        pipx install nox
+        pipx inject nox nox-poetry
+
+    - name: Run mypy
+      run: |
+        nox -rs mypy

docs/src/_contribute/style.md

@@ -22,7 +22,8 @@ Python:
 Flake8 is a python tool that glues together `pycodestyle`, `pyflakes`, `mccabe`, and third-party plugins to check the style and quality of python code,
 and `wemake-python-styleguide` is a plugin for Flake8 that offers an extensive set of opinionated rules that encourage clean and correct code.
 
-[MyPy is currently not executed in CI](https://github.com/meltano/meltano/issues/6491). It currently raises many issues when run. We intend to address them over time.
+[MyPy is being adopted incrementally by this codebase](https://github.com/meltano/meltano/issues/6491). If you are adding new code, please use type hints.
+To enable type checks for an existing module, _de-glob_ it in the whitelist in `pyproject.toml` and fix any errors.
 
 Javascript:
 

noxfile.py

@@ -82,3 +82,17 @@ def coverage(session: Session) -> None:
         session.run("coverage", "combine")
 
     session.run("coverage", *args)
+
+
+@nox_session(python=main_python_version)
+def mypy(session: Session) -> None:
+    """Run mypy type checking.
+
+    Args:
+        session: Nox session.
+    """
+    args = session.posargs or ["src/meltano"]
+
+    session.install(".")
+    session.install("mypy", "sqlalchemy2-stubs")
+    session.run("mypy", *args)

pyproject.toml

@@ -156,6 +156,84 @@ version_files = [
     'docs/package.json:^  "version":',
 ]
 
+[tool.mypy]
+files = "src"
+
+# The following whitelist is used to allow for incremental adoption
+# of MyPy. Modules should be removed from this whitelist as and when
+# their respective type errors have been addressed. No new modules
+# should be added to this whitelist.
+[[tool.mypy.overrides]]
+module = [
+  "meltano.api.*",
+  "meltano.cli.*",
+  "meltano.migrations.*",
+  "meltano.oauth.*",
+  # Way too many modules at the root of meltano.core to tackle them all at once
+  # so listing them individually here.
+  "meltano.core.cli_messages",
+  "meltano.core.config_service",
+  "meltano.core.connection_service",
+  "meltano.core.db",
+  "meltano.core.discovery_file",
+  "meltano.core.elt_context",
+  "meltano.core.environment",
+  "meltano.core.environment_service",
+  "meltano.core.error",
+  "meltano.core.extract_utils",
+  "meltano.core.extractor",
+  "meltano.core.meltano_file",
+  "meltano.core.meltano_invoker",
+  "meltano.core.migration_service",
+  "meltano.core.models",
+  "meltano.core.plugin_discovery_service",
+  "meltano.core.plugin_install_service",
+  "meltano.core.plugin_invoker",
+  "meltano.core.plugin_location_remove",
+  "meltano.core.plugin_lock_service",
+  "meltano.core.plugin_remove_service",
+  "meltano.core.plugin_test_service",
+  "meltano.core.project",
+  "meltano.core.project_add_service",
+  "meltano.core.project_files",
+  "meltano.core.project_init_service",
+  "meltano.core.project_plugins_service",
+  "meltano.core.project_settings_service",
+  "meltano.core.schedule",
+  "meltano.core.schedule_service",
+  "meltano.core.select_service",
+  "meltano.core.setting",
+  "meltano.core.setting_definition",
+  "meltano.core.settings_service",
+  "meltano.core.settings_store",
+  "meltano.core.sqlalchemy",
+  "meltano.core.state_service",
+  "meltano.core.task_sets",
+  "meltano.core.task_sets_service",
+  "meltano.core.transform_add_service",
+  "meltano.core.upgrade_service",
+  "meltano.core.validation_service",
+  "meltano.core.venv_service",
+  "meltano.core.yaml",
+  # Meltano Core submodules
+  "meltano.core.behavior.*",
+  "meltano.core.block.*",
+  "meltano.core.bundle.*",
+  "meltano.core.compiler.*",
+  "meltano.core.container.*",
+  "meltano.core.hub.*",
+  "meltano.core.job.*",
+  "meltano.core.legacy_tracking.*",
+  "meltano.core.logging.*",
+  "meltano.core.m5o.*",
+  "meltano.core.plugin.*",
+  "meltano.core.runner.*",
+  "meltano.core.sql.*",
+  "meltano.core.tracking.*",
+  "meltano.core.utils.*",
+]
+ignore_errors = true
+
 [build-system]
 requires = ["poetry-core>=1.0.0"]
 build-backend = "poetry.core.masonry.api"

src/meltano/__init__.py

@@ -3,5 +3,5 @@
 
 from __future__ import annotations
 
-# Managed by bumpversion
+# Managed by commitizen
 __version__ = "2.5.0"

src/meltano/core/container/__init__.py

@@ -0,0 +1,3 @@
+"""Handling of plugin container commands."""
+
+from __future__ import annotations

src/meltano/core/container/container_service.py

@@ -8,7 +8,7 @@
 
 from structlog.stdlib import get_logger
 
-from .container_spec import ContainerSpec
+from meltano.core.container.container_spec import ContainerSpec
 
 if TYPE_CHECKING:
     from aiodocker.containers import DockerContainer

src/meltano/migrations/utils/__init__.py

@@ -0,0 +1,3 @@
+"""SQL migration utilities."""
+
+from __future__ import annotations