ENH: implement editable loader with PEP 829 .start files

on Python 3.15 and later. Editable wheels are Python version specific,
thus there is no need to write both a .pth and a .start file for
Python versions that support the latter.

Editable wheels implemented with .pth files just imported the
implementation module and relied on the execution of the module body
on import for the installation of the module loader.  PEP 829 .start
files need to specify a callable that takes no arguments. To keep the
difference between .pth and .start file implementations to a minimum,
define such callable in the implementation module and switch the .pth
file to explicitly call it.

Fixes #846

Author: dnicolodi

mesonpy/__init__.py

@@ -553,18 +553,28 @@ def build(self, directory: Path, source_dir: pathlib.Path, build_dir: pathlib.Pa
             whl.writestr(
                 f'{loader_module_name}.py',
                 importlib.resources.files('mesonpy').joinpath('_editable.py').read_bytes() + textwrap.dedent(f'''
-                   install(
-                       {self._metadata.name!r},
-                       {self._top_level_modules!r},
-                       {os.fspath(build_dir)!r},
-                       {build_command!r},
-                       {verbose!r},
-                   )''').encode('utf-8'))
-
-            # install .pth file
-            whl.writestr(
-                f'{self._metadata.canonical_name}-editable.pth',
-                f'import {loader_module_name}'.encode('utf-8'))
+                   def init():
+                       finder = MesonpyMetaFinder(
+                           {self._metadata.name!r},
+                           {self._top_level_modules!r},
+                           {os.fspath(build_dir)!r},
+                           {build_command!r},
+                           {verbose!r},
+                       )
+                       sys.meta_path.insert(0, finder)
+                       sys.path_hooks.insert(0, finder._path_hook)
+                   ''').encode('utf-8'))
+
+            if sys.version_info >= (3, 15):
+                # install .start file
+                whl.writestr(
+                    f'{self._metadata.canonical_name}-editable.start',
+                    f'{loader_module_name}:init'.encode('utf-8'))
+            else:
+                # install .pth file
+                whl.writestr(
+                    f'{self._metadata.canonical_name}-editable.pth',
+                    f'import {loader_module_name}; {loader_module_name}.init()'.encode('utf-8'))
 
         return wheel_file
 

mesonpy/_editable.py

@@ -410,9 +410,3 @@ def iter_modules(self, prefix: str) -> Iterator[Tuple[str, bool]]:
             elif modname and '.' not in modname:
                 yielded.add(modname)
                 yield prefix + modname, False
-
-
-def install(package: str, names: Set[str], path: str, cmd: List[str], verbose: bool) -> None:
-    finder = MesonpyMetaFinder(package, names, path, cmd, verbose)
-    sys.meta_path.insert(0, finder)
-    sys.path_hooks.insert(0, finder._path_hook)

tests/test_editable.py

@@ -13,6 +13,7 @@
 from contextlib import redirect_stdout
 
 import pytest
+import wheel.wheelfile
 
 import mesonpy
 
@@ -190,6 +191,21 @@ def test_editable_install(venv, editable_simple):
     assert venv.python('-c', 'import simple; print(simple.data())').strip() == 'ABC'
 
 
+def test_editable_contents(editable_simple):
+    artifact = wheel.wheelfile.WheelFile(editable_simple)
+
+    impl = 'start' if sys.version_info >= (3, 15) else 'pth'
+    expecting = {
+        'simple-1.0.0.dist-info/METADATA',
+        'simple-1.0.0.dist-info/RECORD',
+        'simple-1.0.0.dist-info/WHEEL',
+        '_simple_editable_loader.py',
+        f'simple-editable.{impl}',
+    }
+
+    assert set(artifact.namelist()) == expecting
+
+
 def test_editble_reentrant(venv, editable_imports_itself_during_build):
     venv.pip('install', os.fspath(editable_imports_itself_during_build))
     assert venv.python('-c', 'import plat; print(plat.data())').strip() == 'ABC'