feat: support PEP 723 with a toml load function

.pre-commit-config.yaml

@@ -40,6 +40,7 @@ repos:
           - jinja2
           - packaging
           - importlib_metadata
+          - tomli
           - uv
 
   - repo: https://github.com/codespell-project/codespell

docs/tutorial.rst

@@ -169,6 +169,46 @@ dependency (e.g. ``libfoo``) is available during installation:
 These commands will run even if you are only installing, and will not run if
 the environment is being reused without reinstallation.
 
+
+Loading dependencies from pyproject.toml or scripts
+---------------------------------------------------
+
+One common need is loading a dependency list from a ``pyproject.toml`` file
+(say to prepare an environment without installing the package) or supporting
+`PEP 723 <https://peps.python.org/pep-0723>`_ scripts. Nox provides a helper to
+load these with ``nox.toml.load``. It can be passed a filepath to a toml file
+or to a script file following PEP 723. For example, if you have the following
+``peps.py``:
+
+
+.. code-block:: python
+
+    # /// script
+    # requires-python = ">=3.11"
+    # dependencies = [
+    #   "requests<3",
+    #   "rich",
+    # ]
+    # ///
+
+    import requests
+    from rich.pretty import pprint
+
+    resp = requests.get("https://peps.python.org/api/peps.json")
+    data = resp.json()
+    pprint([(k, v["title"]) for k, v in data.items()][:10])
+
+You can make a session for it like this:
+
+.. code-block:: python
+
+   @nox.session
+   def peps(session):
+       requirements = nox.toml.load("peps.py")["dependencies"]
+       session.install(*requirements)
+       session.run("peps.py")
+
+
 Running commands
 ----------------
 

nox/__init__.py

@@ -14,6 +14,7 @@
 
 from __future__ import annotations
 
+from nox import toml
 from nox._options import noxfile_options as options
 from nox._parametrize import Param as param
 from nox._parametrize import parametrize_decorator as parametrize
@@ -22,4 +23,12 @@
 
 needs_version: str | None = None
 
-__all__ = ["needs_version", "parametrize", "param", "session", "options", "Session"]
+__all__ = [
+    "needs_version",
+    "parametrize",
+    "param",
+    "session",
+    "options",
+    "Session",
+    "toml",
+]

nox/toml.py

@@ -0,0 +1,69 @@
+from __future__ import annotations
+
+import os
+import re
+import sys
+from pathlib import Path
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from typing import Any
+
+if sys.version_info < (3, 11):
+    import tomli as tomllib
+else:
+    import tomllib
+
+
+__all__ = ["load"]
+
+
+def __dir__() -> list[str]:
+    return __all__
+
+
+# Note: the implementation (including this regex) taken from PEP 723
+# https://peps.python.org/pep-0723
+
+REGEX = re.compile(
+    r"(?m)^# /// (?P<type>[a-zA-Z0-9-]+)$\s(?P<content>(^#(| .*)$\s)+)^# ///$"
+)
+
+
+def load(filename: os.PathLike[str] | str) -> dict[str, Any]:
+    """
+    Load a toml file or a script with a PEP 723 script block.
+
+    The file must have a ``.toml`` extension to be considered a toml file or a
+    ``.py`` extension / no extension to be considered a script. Other file
+    extensions are not valid in this function.
+    """
+    filepath = Path(filename)
+    if filepath.suffix == ".toml":
+        return _load_toml_file(filepath)
+    if filepath.suffix in {".py", ""}:
+        return _load_script_block(filepath)
+    msg = f"Extension must be .py or .toml, got {filepath.suffix}"
+    raise ValueError(msg)
+
+
+def _load_toml_file(filepath: Path) -> dict[str, Any]:
+    with filepath.open("rb") as f:
+        return tomllib.load(f)
+
+
+def _load_script_block(filepath: Path) -> dict[str, Any]:
+    name = "script"
+    script = filepath.read_text(encoding="utf-8")
+    matches = list(filter(lambda m: m.group("type") == name, REGEX.finditer(script)))
+
+    if not matches:
+        raise ValueError(f"No {name} block found in {filepath}")
+    if len(matches) > 1:
+        raise ValueError(f"Multiple {name} blocks found in {filepath}")
+
+    content = "".join(
+        line[2:] if line.startswith("# ") else line[1:]
+        for line in matches[0].group("content").splitlines(keepends=True)
+    )
+    return tomllib.loads(content)

pyproject.toml

@@ -44,6 +44,7 @@ dependencies = [
   "colorlog<7.0.0,>=2.6.1",
   'importlib-metadata; python_version < "3.8"',
   "packaging>=20.9",
+  'tomli>=1; python_version < "3.11"',
   'typing-extensions>=3.7.4; python_version < "3.8"',
   "virtualenv>=20.14.1",
 ]

tests/test_toml.py

@@ -0,0 +1,107 @@
+import textwrap
+from pathlib import Path
+
+import pytest
+
+import nox
+
+
+def test_load_pyproject(tmp_path: Path) -> None:
+    filepath = tmp_path / "example.toml"
+    filepath.write_text(
+        """
+        [project]
+        name = "hi"
+        version = "1.0"
+        dependencies = ["numpy", "requests"]
+        """
+    )
+
+    toml = nox.toml.load(filepath)
+    assert toml["project"]["dependencies"] == ["numpy", "requests"]
+
+
+@pytest.mark.parametrize("example", ["example.py", "example"])
+def test_load_script_block(tmp_path: Path, example: str) -> None:
+    filepath = tmp_path / example
+    filepath.write_text(
+        textwrap.dedent(
+            """\
+            #!/usr/bin/env pipx run
+            # /// script
+            # requires-python = ">=3.11"
+            # dependencies = [
+            #   "requests<3",
+            #   "rich",
+            # ]
+            # ///
+
+            import requests
+            from rich.pretty import pprint
+
+            resp = requests.get("https://peps.python.org/api/peps.json")
+            data = resp.json()
+            pprint([(k, v["title"]) for k, v in data.items()][:10])
+            """
+        )
+    )
+
+    toml = nox.toml.load(filepath)
+    assert toml["dependencies"] == ["requests<3", "rich"]
+
+
+def test_load_no_script_block(tmp_path: Path) -> None:
+    filepath = tmp_path / "example.py"
+    filepath.write_text(
+        textwrap.dedent(
+            """\
+            #!/usr/bin/python
+
+            import requests
+            from rich.pretty import pprint
+
+            resp = requests.get("https://peps.python.org/api/peps.json")
+            data = resp.json()
+            pprint([(k, v["title"]) for k, v in data.items()][:10])
+            """
+        )
+    )
+
+    with pytest.raises(ValueError, match="No script block found"):
+        nox.toml.load(filepath)
+
+
+def test_load_multiple_script_block(tmp_path: Path) -> None:
+    filepath = tmp_path / "example.py"
+    filepath.write_text(
+        textwrap.dedent(
+            """\
+            # /// script
+            # dependencies = [
+            #   "requests<3",
+            #   "rich",
+            # ]
+            # ///
+
+            # /// script
+            # requires-python = ">=3.11"
+            # ///
+
+            import requests
+            from rich.pretty import pprint
+
+            resp = requests.get("https://peps.python.org/api/peps.json")
+            data = resp.json()
+            pprint([(k, v["title"]) for k, v in data.items()][:10])
+            """
+        )
+    )
+
+    with pytest.raises(ValueError, match="Multiple script blocks found"):
+        nox.toml.load(filepath)
+
+
+def test_load_non_recongnised_extension():
+    msg = "Extension must be .py or .toml, got .txt"
+    with pytest.raises(ValueError, match=msg):
+        nox.toml.load("some.txt")