Optional metadata sans distutils

setuptools/config/_apply_pyprojecttoml.py

@@ -171,21 +171,7 @@ def _people(dist: "Distribution", val: List[dict], _root_dir: _Path, kind: str):
 
 
 def _project_urls(dist: "Distribution", val: dict, _root_dir):
-    special = {"downloadurl": "download_url", "homepage": "url"}
-    for key, url in val.items():
-        norm_key = json_compatible_key(key).replace("_", "")
-        _set_config(dist, special.get(norm_key, key), url)
-    # If `homepage` is missing, distutils will warn the following message:
-    #     "warning: check: missing required meta-data: url"
-    # In the context of PEP 621, users might ask themselves: "which url?".
-    # Let's add a warning before distutils check to help users understand the problem:
-    if not dist.metadata.url:
-        msg = (
-            "Missing `Homepage` url.\nIt is advisable to link some kind of reference "
-            "for your project (e.g. source code or documentation).\n"
-        )
-        _logger.warning(msg)
-    _set_config(dist, "project_urls", val.copy())
+    _set_config(dist, "project_urls", val)
 
 
 def _python_requires(dist: "Distribution", val: dict, _root_dir):

setuptools/dist.py

@@ -102,7 +102,7 @@ def _read_list_from_msg(msg: "Message", field: str) -> Optional[List[str]]:
 
 def _read_payload_from_msg(msg: "Message") -> Optional[str]:
     value = msg.get_payload().strip()
-    if value == 'UNKNOWN':
+    if value == 'UNKNOWN' or not value:
         return None
     return value
 
@@ -174,7 +174,10 @@ def write_field(key, value):
     write_field('Metadata-Version', str(version))
     write_field('Name', self.get_name())
     write_field('Version', self.get_version())
-    write_field('Summary', single_line(self.get_description()))
+
+    summary = self.get_description()
+    if summary:
+        write_field('Summary', single_line(summary))
 
     optional_fields = (
         ('Home-page', 'url'),
@@ -190,16 +193,19 @@ def write_field(key, value):
         if attr_val is not None:
             write_field(field, attr_val)
 
-    license = rfc822_escape(self.get_license())
-    write_field('License', license)
+    license = self.get_license()
+    if license:
+        write_field('License', rfc822_escape(license))
+
     for project_url in self.project_urls.items():
         write_field('Project-URL', '%s, %s' % project_url)
 
     keywords = ','.join(self.get_keywords())
     if keywords:
         write_field('Keywords', keywords)
 
-    for platform in self.get_platforms():
+    platforms = self.get_platforms() or []
+    for platform in platforms:
         write_field('Platform', platform)
 
     self._write_list(file, 'Classifier', self.get_classifiers())
@@ -222,7 +228,11 @@ def write_field(key, value):
 
     self._write_list(file, 'License-File', self.license_files or [])
 
-    file.write("\n%s\n\n" % self.get_long_description())
+    long_description = self.get_long_description()
+    if long_description:
+        file.write("\n%s" % long_description)
+        if not long_description.endswith("\n"):
+            file.write("\n")
 
 
 sequence = tuple, list

setuptools/tests/config/test_apply_pyprojecttoml.py

@@ -298,19 +298,26 @@ def test_example_file_not_in_wheel(self, setuptools_wheel):
 def core_metadata(dist) -> str:
     with io.StringIO() as buffer:
         dist.metadata.write_pkg_file(buffer)
-        value = "\n".join(buffer.getvalue().strip().splitlines())
+        pkg_file_txt = buffer.getvalue()
 
+    skip_prefixes = ()
+    skip_lines = set()
     # ---- DIFF NORMALISATION ----
     # PEP 621 is very particular about author/maintainer metadata conversion, so skip
-    value = re.sub(r"^(Author|Maintainer)(-email)?:.*$", "", value, flags=re.M)
+    skip_prefixes += ("Author:", "Author-email:", "Maintainer:", "Maintainer-email:")
     # May be redundant with Home-page
-    value = re.sub(r"^Project-URL: Homepage,.*$", "", value, flags=re.M)
+    skip_prefixes += ("Project-URL: Homepage,", "Home-page:")
     # May be missing in original (relying on default) but backfilled in the TOML
-    value = re.sub(r"^Description-Content-Type:.*$", "", value, flags=re.M)
+    skip_prefixes += ("Description-Content-Type:",)
     # ini2toml can automatically convert `tests_require` to `testing` extra
-    value = value.replace("Provides-Extra: testing\n", "")
+    skip_lines.add("Provides-Extra: testing")
     # Remove empty lines
-    value = re.sub(r"^\s*$", "", value, flags=re.M)
-    value = re.sub(r"^\n", "", value, flags=re.M)
+    skip_lines.add("")
 
-    return value
+    result = []
+    for line in pkg_file_txt.splitlines():
+        if line.startswith(skip_prefixes) or line in skip_lines:
+            continue
+        result.append(line + "\n")
+
+    return "".join(result)