Use default encoding when creating .pth files in editable_wheel (#4009)

newsfragments/4009.misc.rst

@@ -0,0 +1 @@
+Use default encoding to create ``.pth`` files with ``editable_wheel``.

setuptools/command/editable_wheel.py

@@ -11,6 +11,7 @@
 """
 
 import logging
+import io
 import os
 import shutil
 import sys
@@ -401,7 +402,7 @@ def __init__(self, dist: Distribution, name: str, path_entries: List[Path]):
 
     def __call__(self, wheel: "WheelFile", files: List[str], mapping: Dict[str, str]):
         entries = "\n".join((str(p.resolve()) for p in self.path_entries))
-        contents = bytes(f"{entries}\n", "utf-8")
+        contents = _encode_pth(f"{entries}\n")
         wheel.writestr(f"__editable__.{self.name}.pth", contents)
 
     def __enter__(self):
@@ -509,7 +510,7 @@ def __call__(self, wheel: "WheelFile", files: List[str], mapping: Dict[str, str]
         content = bytes(_finder_template(name, roots, namespaces_), "utf-8")
         wheel.writestr(f"{finder}.py", content)
 
-        content = bytes(f"import {finder}; {finder}.install()", "utf-8")
+        content = _encode_pth(f"import {finder}; {finder}.install()")
         wheel.writestr(f"__editable__.{self.name}.pth", content)
 
     def __enter__(self):
@@ -525,6 +526,24 @@ def __exit__(self, _exc_type, _exc_value, _traceback):
         InformationOnly.emit("Editable installation.", msg)
 
 
+def _encode_pth(content: str) -> bytes:
+    """.pth files are always read with 'locale' encoding, the recommendation
+    from the cpython core developers is to write them as ``open(path, "w")``
+    and ignore warnings (see python/cpython#77102, pypa/setuptools#3937).
+    This function tries to simulate this behaviour without having to create an
+    actual file, in a way that supports a range of active Python versions.
+    (There seems to be some variety in the way different version of Python handle
+    ``encoding=None``, not all of them use ``locale.getpreferredencoding(False)``).
+    """
+    encoding = "locale" if sys.version_info >= (3, 10) else None
+    with io.BytesIO() as buffer:
+        wrapper = io.TextIOWrapper(buffer, encoding)
+        wrapper.write(content)
+        wrapper.flush()
+        buffer.seek(0)
+        return buffer.read()
+
+
 def _can_symlink_files(base_dir: Path) -> bool:
     with TemporaryDirectory(dir=str(base_dir.resolve())) as tmp:
         path1, path2 = Path(tmp, "file1.txt"), Path(tmp, "file2.txt")

setuptools/tests/test_editable_install.py

@@ -22,6 +22,7 @@
 from setuptools.command.editable_wheel import (
     _DebuggingTips,
     _LinkTree,
+    _encode_pth,
     _find_virtual_namespaces,
     _find_namespaces,
     _find_package_roots,
@@ -1107,6 +1108,13 @@ def test_debugging_tips(tmpdir_cwd, monkeypatch):
         cmd.run()
 
 
+@pytest.mark.filterwarnings("error")
+def test_encode_pth():
+    """Ensure _encode_pth function does not produce encoding warnings"""
+    content = _encode_pth("tkmilan_ç_utf8")  # no warnings (would be turned into errors)
+    assert isinstance(content, bytes)
+
+
 def install_project(name, venv, tmp_path, files, *opts):
     project = tmp_path / name
     project.mkdir()