Fix problem with dynamic readme (#3247)

changelog.d/3247.misc.rst

@@ -0,0 +1,2 @@
+Fixed problem preventing ``readme`` specified as dynamic in ``pyproject.toml``
+from being dynamically specified in ``setup.py``.

setuptools/config/pyprojecttoml.py

@@ -316,12 +316,17 @@ def _obtain_version(self, dist: "Distribution", package_dir: Mapping[str, str]):
         return None
 
     def _obtain_readme(self, dist: "Distribution") -> Optional[Dict[str, str]]:
-        if "readme" in self.dynamic:
-            dynamic_cfg = self.dynamic_cfg
+        if "readme" not in self.dynamic:
+            return None
+
+        dynamic_cfg = self.dynamic_cfg
+        if "readme" in dynamic_cfg:
             return {
                 "text": self._obtain(dist, "readme", {}),
                 "content-type": dynamic_cfg["readme"].get("content-type", "text/x-rst"),
             }
+
+        self._ensure_previously_set(dist, "readme")
         return None
 
     def _obtain_entry_points(

setuptools/tests/config/test_pyprojecttoml.py

@@ -253,6 +253,20 @@ def test_dynamic_without_config(self, tmp_path):
         with pytest.raises(OptionError, match="No configuration .* .classifiers."):
             read_configuration(pyproject)
 
+    def test_dynamic_readme_from_setup_script_args(self, tmp_path):
+        config = """
+        [project]
+        name = "myproj"
+        version = '42'
+        dynamic = ["readme"]
+        """
+        pyproject = tmp_path / "pyproject.toml"
+        pyproject.write_text(cleandoc(config))
+        dist = Distribution(attrs={"long_description": "42"})
+        # No error should occur because of missing `readme`
+        dist = apply_configuration(dist, pyproject)
+        assert dist.metadata.long_description == "42"
+
     def test_dynamic_without_file(self, tmp_path):
         config = """
         [project]