packages: Add tool to build an editable package impurely

This is to be used with build-systems such as `meson-python` that builds native extensions.
It's purpose is the same as that of `python setup.py build_ext -i`, just using PEP-660.

Of course this is not a perfect analogy to using something like `meson-python` directly.
We just use the `.pth` machinery directly, while `meson-python` uses a more complex hook machinery:

- Automatic inference of Python

`meson-python` puts the editable build in `build/cp312`.
Users will have to manually override any automatically inferred editable root to point to the correct build directory.

- Automatic rebuilds

Thanks to it's hook machinery `meson-python` can rebuild packages as necessary on load.
This automatic rebuild behaviour will not work, and users will have to call `build-editable` manually.

These are just two examples of build-system specific behaviour that will not work.

doc/src/SUMMARY.md

@@ -45,6 +45,9 @@
     - [resolvers](./build/lib/resolvers.nix)
   - [hooks](./build/hooks/default.nix)
 
+- [Packages](./packages.md)
+  - [build-editable](./packages/build-editable/README.md)
+
 # Contributing
 
 - [Hacking](./HACKING.md)

packages/build-editable/.python-version

@@ -0,0 +1 @@
+3.12

packages/build-editable/README.md

@@ -0,0 +1,23 @@
+# build-editable
+
+This package is intended to aid Nix-based Python development using [editable installs](https://pip.pypa.io/en/latest/topics/local-project-installs/#editable-installs).
+
+This is to be used with build-systems such as `meson-python` that builds native extensions.
+It's purpose is similar to that of `python setup.py build_ext -i`, just using PEP-660.
+
+## Problems
+
+Of course this is not a perfect analogy to using something like `meson-python` directly.
+We just use the `.pth` machinery directly, while `meson-python` uses a more complex hook machinery:
+
+- Automatic inference of Python
+
+  `meson-python` puts the editable build in `build/cp312`.
+  Users will have to manually override any editable root automatically inferred by Nix to point to the correct build directory.
+
+- Automatic rebuilds
+
+  Thanks to it's hook machinery `meson-python` can rebuild packages as necessary on load.
+  This automatic rebuild behaviour will not work, and users will have to call `build-editable` manually.
+
+These are just two examples of build-system specific behaviour that will not work out of the box.

packages/build-editable/pyproject.toml

@@ -0,0 +1,17 @@
+[project]
+name = "build-editable"
+version = "0.1.0"
+description = "Build editable package"
+readme = "README.md"
+requires-python = ">=3.11"
+dependencies = [
+    "editables>=0.5",
+    "pyproject-hooks>=1.1.0",
+]
+
+[project.scripts]
+build-editable = "build_editable:build-editable"
+
+[build-system]
+requires = ["hatchling"]
+build-backend = "hatchling.build"

packages/build-editable/src/build_editable/__init__.py

@@ -0,0 +1,49 @@
+import argparse
+import os
+import os.path
+import tempfile
+from pathlib import Path
+
+import pyproject_hooks
+import tomllib
+
+argparser = argparse.ArgumentParser(
+    description="Trigger in-place builds required for building editable packages with native extensions"
+)
+_ = argparser.add_argument(
+    "--wheel-dir",
+    help="Output wheel to directory instead of a temporary directory that gets immediately deleted",
+    type=str,
+)
+
+
+def main() -> None:
+    args = argparser.parse_args()
+
+    cwd = Path(os.getcwd())
+    if args.wheel_dir is not None:
+        out: str = str(args.wheel_dir)
+    else:
+        out_temp = tempfile.TemporaryDirectory()
+        out = out_temp.name
+
+    with open(cwd.joinpath("pyproject.toml"), "rb") as pyproject_file:
+        pyproject = tomllib.load(pyproject_file)
+
+    # Get build backend with fallback behaviour
+    # https://pip.pypa.io/en/stable/reference/build-system/pyproject-toml/#fallback-behaviour
+    try:
+        build_backend: str = pyproject["build-system"]["build-backend"]
+    except KeyError:
+        build_backend = "setuptools.build_meta:__legacy__"
+
+    # Call editable build hooks using pyproject-hooks
+    hook_caller = pyproject_hooks.BuildBackendHookCaller(
+        source_dir=cwd,
+        build_backend=build_backend,
+    )
+    hook_caller.build_editable(out)
+
+
+if __name__ == "__main__":
+    main()