Add nox as the task runner

- set up skeletons for all planned nox sessions
- provide a base configuration for all nox sessions
- create two utility functions:
  + _begin() => run commands common to all sessions
  + _install_packages() => install dependencies in nox sessions
                           respecting the versions pinned in poetry.lock
- add the following file:
  + noxfile.py

.gitignore

@@ -1,3 +1,4 @@
+.cache/
 *.egg-info/
 .python-version
 .venv/

noxfile.py

@@ -0,0 +1,114 @@
+"""Configure nox for the isolated test and lint environments."""
+
+import os
+import tempfile
+from typing import Any
+
+import nox
+from nox.sessions import Session
+
+
+MAIN_PYTHON = '3.8'
+
+# Keep the project is forward compatible.
+NEXT_PYTHON = '3.9'
+
+# Paths with *.py files.
+SRC_LOCATIONS = 'noxfile.py', 'src/'
+
+
+# Use a unified .cache/ folder for all tools.
+nox.options.envdir = '.cache/nox'
+
+# All tools except git and poetry are project dependencies.
+# Avoid accidental successes if the environment is not set up properly.
+nox.options.error_on_external_run = True
+
+# Run only CI related checks by default.
+nox.options.sessions = (
+    'format',
+    'lint',
+    f'test-{MAIN_PYTHON}',
+    f'test-{NEXT_PYTHON}',
+)
+
+
+@nox.session(name='format', python=MAIN_PYTHON)
+def format_(session: Session):
+    """Format source files."""
+    _begin(session)
+
+
+@nox.session(python=MAIN_PYTHON)
+def lint(session: Session):
+    """Lint source files."""
+    _begin(session)
+
+
+@nox.session(python=[MAIN_PYTHON, NEXT_PYTHON])
+def test(session: Session):
+    """Test the code base."""
+    _begin(session)
+
+
+@nox.session(name='pre-commit', python=MAIN_PYTHON, venv_backend='none')
+def pre_commit(session: Session):
+    """Source files must be well-formed before they enter git."""
+    _begin(session)
+    session.notify('format')
+    session.notify('lint')
+
+
+@nox.session(name='pre-merge', python=MAIN_PYTHON, venv_backend='none')
+def pre_merge(session: Session):
+    """The test suite must pass before merges are made."""
+    _begin(session)
+    session.notify('test')
+
+
+def _begin(session: Session) -> None:
+    """Show generic info about a session."""
+    if session.posargs:
+        print('extra arguments:', *session.posargs)
+
+    session.run('python', '--version')
+
+    # Fake GNU's pwd.
+    session.log('pwd')
+    print(os.getcwd())
+
+
+def _install_packages(
+    session: Session, *packages_or_pip_args: str, **kwargs: Any
+) -> None:
+    """Install packages respecting the poetry.lock file.
+
+    This function wraps nox.sessions.Session.install() such that it installs
+    packages respecting the pinnned versions specified in poetry's lock file.
+    This makes nox sessions even more deterministic.
+
+    Args:
+        session: the Session object
+        *packages_or_pip_args: the packages to be installed or pip options
+        **kwargs: passed on to nox.sessions.Session.install()
+    """
+    if session.virtualenv.reuse_existing:
+        session.log(
+            'No dependencies are installed as an existing environment is re-used',
+        )
+        return
+
+    session.log('Dependencies are installed respecting the poetry.lock file')
+
+    with tempfile.NamedTemporaryFile() as requirements_txt:
+        session.run(
+            'poetry',
+            'export',
+            '--dev',
+            '--format=requirements.txt',
+            f'--output={requirements_txt.name}',
+            external=True,
+        )
+        session.install(
+            f'--constraint={requirements_txt.name}', *packages_or_pip_args, **kwargs,
+        )

poetry.lock

@@ -1,8 +1,152 @@
-package = []
+[[package]]
+category = "dev"
+description = "A small Python module for determining appropriate platform-specific dirs, e.g. a \"user data dir\"."
+name = "appdirs"
+optional = false
+python-versions = "*"
+version = "1.4.4"
+
+[[package]]
+category = "dev"
+description = "Bash tab completion for argparse"
+name = "argcomplete"
+optional = false
+python-versions = "*"
+version = "1.12.0"
+
+[package.extras]
+test = ["coverage", "flake8", "pexpect", "wheel"]
+
+[[package]]
+category = "dev"
+description = "Cross-platform colored terminal text."
+marker = "sys_platform == \"win32\""
+name = "colorama"
+optional = false
+python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
+version = "0.4.3"
+
+[[package]]
+category = "dev"
+description = "Log formatting with colors!"
+name = "colorlog"
+optional = false
+python-versions = "*"
+version = "4.2.1"
+
+[package.dependencies]
+colorama = "*"
+
+[[package]]
+category = "dev"
+description = "Distribution utilities"
+name = "distlib"
+optional = false
+python-versions = "*"
+version = "0.3.1"
+
+[[package]]
+category = "dev"
+description = "A platform independent file lock."
+name = "filelock"
+optional = false
+python-versions = "*"
+version = "3.0.12"
+
+[[package]]
+category = "dev"
+description = "Flexible test automation."
+name = "nox"
+optional = false
+python-versions = ">=3.5"
+version = "2020.5.24"
+
+[package.dependencies]
+argcomplete = ">=1.9.4,<2.0"
+colorlog = ">=2.6.1,<5.0.0"
+py = ">=1.4.0,<2.0.0"
+virtualenv = ">=14.0.0"
+
+[package.extras]
+tox_to_nox = ["jinja2", "tox"]
+
+[[package]]
+category = "dev"
+description = "library with cross-python path, ini-parsing, io, code, log facilities"
+name = "py"
+optional = false
+python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
+version = "1.9.0"
+
+[[package]]
+category = "dev"
+description = "Python 2 and 3 compatibility utilities"
+name = "six"
+optional = false
+python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*"
+version = "1.15.0"
+
+[[package]]
+category = "dev"
+description = "Virtual Python Environment builder"
+name = "virtualenv"
+optional = false
+python-versions = "!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,>=2.7"
+version = "20.0.29"
+
+[package.dependencies]
+appdirs = ">=1.4.3,<2"
+distlib = ">=0.3.1,<1"
+filelock = ">=3.0.0,<4"
+six = ">=1.9.0,<2"
+
+[package.extras]
+docs = ["proselint (>=0.10.2)", "sphinx (>=3)", "sphinx-argparse (>=0.2.5)", "sphinx-rtd-theme (>=0.4.3)", "towncrier (>=19.9.0rc1)"]
+testing = ["coverage (>=5)", "coverage-enable-subprocess (>=1)", "flaky (>=3)", "pytest (>=4)", "pytest-env (>=0.6.2)", "pytest-freezegun (>=0.4.1)", "pytest-mock (>=2)", "pytest-randomly (>=1)", "pytest-timeout (>=1)", "pytest-xdist (>=1.31.0)", "packaging (>=20.0)", "xonsh (>=0.9.16)"]
 
 [metadata]
-content-hash = "fafb334cb038533f851c23d0b63254223abf72ce4f02987e7064b0c95566699a"
+content-hash = "4ff12f695975572453949608224e06fb36641151ee69668f8603f7bc3435a51f"
 lock-version = "1.0"
 python-versions = "^3.8"
 
 [metadata.files]
+appdirs = [
+    {file = "appdirs-1.4.4-py2.py3-none-any.whl", hash = "sha256:a841dacd6b99318a741b166adb07e19ee71a274450e68237b4650ca1055ab128"},
+    {file = "appdirs-1.4.4.tar.gz", hash = "sha256:7d5d0167b2b1ba821647616af46a749d1c653740dd0d2415100fe26e27afdf41"},
+]
+argcomplete = [
+    {file = "argcomplete-1.12.0-py2.py3-none-any.whl", hash = "sha256:91dc7f9c7f6281d5a0dce5e73d2e33283aaef083495c13974a7dd197a1cdc949"},
+    {file = "argcomplete-1.12.0.tar.gz", hash = "sha256:2fbe5ed09fd2c1d727d4199feca96569a5b50d44c71b16da9c742201f7cc295c"},
+]
+colorama = [
+    {file = "colorama-0.4.3-py2.py3-none-any.whl", hash = "sha256:7d73d2a99753107a36ac6b455ee49046802e59d9d076ef8e47b61499fa29afff"},
+    {file = "colorama-0.4.3.tar.gz", hash = "sha256:e96da0d330793e2cb9485e9ddfd918d456036c7149416295932478192f4436a1"},
+]
+colorlog = [
+    {file = "colorlog-4.2.1-py2.py3-none-any.whl", hash = "sha256:43597fd822ce705190fc997519342fdaaf44b9b47f896ece7aa153ed4b909c74"},
+    {file = "colorlog-4.2.1.tar.gz", hash = "sha256:75e55822c3a3387d721579241e776de2cf089c9ef9528b1f09e8b04d403ad118"},
+]
+distlib = [
+    {file = "distlib-0.3.1-py2.py3-none-any.whl", hash = "sha256:8c09de2c67b3e7deef7184574fc060ab8a793e7adbb183d942c389c8b13c52fb"},
+    {file = "distlib-0.3.1.zip", hash = "sha256:edf6116872c863e1aa9d5bb7cb5e05a022c519a4594dc703843343a9ddd9bff1"},
+]
+filelock = [
+    {file = "filelock-3.0.12-py3-none-any.whl", hash = "sha256:929b7d63ec5b7d6b71b0fa5ac14e030b3f70b75747cef1b10da9b879fef15836"},
+    {file = "filelock-3.0.12.tar.gz", hash = "sha256:18d82244ee114f543149c66a6e0c14e9c4f8a1044b5cdaadd0f82159d6a6ff59"},
+]
+nox = [
+    {file = "nox-2020.5.24-py3-none-any.whl", hash = "sha256:c4509621fead99473a1401870e680b0aadadce5c88440f0532863595176d64c1"},
+    {file = "nox-2020.5.24.tar.gz", hash = "sha256:61a55705736a1a73efbd18d5b262a43d55a1176546e0eb28b29064cfcffe26c0"},
+]
+py = [
+    {file = "py-1.9.0-py2.py3-none-any.whl", hash = "sha256:366389d1db726cd2fcfc79732e75410e5fe4d31db13692115529d34069a043c2"},
+    {file = "py-1.9.0.tar.gz", hash = "sha256:9ca6883ce56b4e8da7e79ac18787889fa5206c79dcc67fb065376cd2fe03f342"},
+]
+six = [
+    {file = "six-1.15.0-py2.py3-none-any.whl", hash = "sha256:8b74bedcbbbaca38ff6d7491d76f2b06b3592611af620f8426e82dddb04a5ced"},
+    {file = "six-1.15.0.tar.gz", hash = "sha256:30639c035cdb23534cd4aa2dd52c3bf48f06e5f4a941509c8bafd8ce11080259"},
+]
+virtualenv = [
+    {file = "virtualenv-20.0.29-py2.py3-none-any.whl", hash = "sha256:8aa9c37b082664dbce2236fa420759c02d64109d8e6013593ad13914718a30fd"},
+    {file = "virtualenv-20.0.29.tar.gz", hash = "sha256:f14a0a98ea4397f0d926cff950361766b6a73cd5975ae7eb259d12919f819a25"},
+]

pyproject.toml

@@ -23,3 +23,5 @@ repository = "https://github.com/webartifex/urban-meal-delivery"
 python = "^3.8"
 
 [tool.poetry.dev-dependencies]
+# Task Runners
+nox = "^2020.5.24"