Honor but deprecate newlines when processing keywords/platforms

distutils/dist.py

@@ -25,6 +25,7 @@
     overload,
 )
 
+from jaraco.text import SeparatedValues
 from packaging.utils import canonicalize_name, canonicalize_version
 
 from ._log import log
@@ -75,6 +76,34 @@ def _ensure_list(value: str | Iterable[str], fieldname) -> str | list[str]:
     return value
 
 
+def _repair_newlines(value: str) -> str:
+    """
+    Repair a ``keywords`` or ``platforms`` value that improperly uses
+    newlines to separate items.
+
+    These fields separate items with commas (and, in the old specification,
+    PEP 345, with spaces). Newlines were never a valid separator and corrupt
+    the generated ``PKG-INFO``/``METADATA`` (pypa/setuptools#4887). As a
+    forgiving measure, values that supply these fields as newline-separated
+    strings (e.g. a triple-quoted string with one item per line) have each
+    line treated as a separate item by replacing newlines with commas, but
+    the behavior is deprecated. Empty items produced by consecutive or
+    trailing separators are dropped by the caller.
+    """
+    if "\n" not in value:
+        return value
+    # A plain warning (UserWarning) rather than DeprecationWarning so that
+    # it's shown by default; DeprecationWarning is suppressed outside of test
+    # runners and this needs to reach setuptools users (cf. distutils.log.Log).
+    warnings.warn(
+        "Newlines are not a valid separator for the 'keywords' and "
+        "'platforms' fields and their use is deprecated. Separate items "
+        "with commas instead. This will raise an error in the future.",
+        stacklevel=2,
+    )
+    return value.replace("\n", ",")
+
+
 class Distribution:
     """The core of the Distutils.  Most of the work hiding behind 'setup'
     is really done within a Distribution instance, which farms the work out
@@ -643,7 +672,8 @@ def finalize_options(self) -> None:
             if value is None:
                 continue
             if isinstance(value, str):
-                value = [elm.strip() for elm in value.split(',')]
+                # SeparatedValues strips whitespace and discards empty items.
+                value = list(SeparatedValues(_repair_newlines(value)))
                 setattr(self.metadata, attr, value)
 
     def _show_help(

distutils/tests/test_dist.py

@@ -217,6 +217,28 @@ def test_finalize_options(self):
         assert dist.metadata.platforms == ['foo bar']
         assert dist.metadata.keywords == ['foo bar']
 
+        # Newlines are an invalid (deprecated) separator; each line is
+        # treated as a separate item (pypa/setuptools#4887).
+        attrs = {
+            'keywords': 'one\ntwo\nthree\nfour',
+            'platforms': 'one\ntwo\nthree\nfour',
+        }
+        with pytest.warns(UserWarning, match="Newlines"):
+            dist = Distribution(attrs=attrs)
+        assert dist.metadata.platforms == ['one', 'two', 'three', 'four']
+        assert dist.metadata.keywords == ['one', 'two', 'three', 'four']
+
+        # Consecutive, leading, or trailing newlines must not produce
+        # empty items (e.g. from a triple-quoted string with blank lines).
+        attrs = {
+            'keywords': '\none two\n\nthree four\n',
+            'platforms': '\none two\n\nthree four\n',
+        }
+        with pytest.warns(UserWarning, match="Newlines"):
+            dist = Distribution(attrs=attrs)
+        assert dist.metadata.platforms == ['one two', 'three four']
+        assert dist.metadata.keywords == ['one two', 'three four']
+
     def test_get_command_packages(self):
         dist = Distribution()
         assert dist.command_packages is None

newsfragments/4887.feature.rst

@@ -0,0 +1 @@
+Newline-separated ``keywords`` and ``platforms``, which are invalid and corrupt the generated metadata (pypa/setuptools#4887), are now handled forgivingly: each line is treated as a separate item and a deprecation warning is emitted. Newlines were never a valid separator for these fields -- the `old specification <https://peps.python.org/pep-0345/>`_ separated items with spaces and the current one uses commas.

pyproject.toml

@@ -27,6 +27,7 @@ dependencies = [
 	"packaging",
 	"jaraco.functools >= 4",
 	"more_itertools",
+	"jaraco.text",
 ]
 dynamic = ["version"]