feat: setting a non-dynamic field is an error

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

pyproject_metadata/__init__.py

@@ -529,10 +529,20 @@ class StandardMetadata:
     """
 
     metadata_version: str | None = None
+    _locked_metadata: bool = False
 
     def __post_init__(self) -> None:
         self.validate()
 
+    def __setattr__(self, name: str, value: Any) -> None:
+        if self._locked_metadata and name.replace('_', '-') not in set(self.dynamic) | {
+            'metadata-version',
+            'dynamic-metadata',
+        }:
+            msg = f'Field "{name}" is not dynamic'
+            raise AttributeError(msg)
+        super().__setattr__(name, value)
+
     def validate(self, *, warn: bool = True) -> None:
         if self.auto_metadata_version not in KNOWN_METADATA_VERSIONS:
             msg = f'The metadata_version must be one of {KNOWN_METADATA_VERSIONS} or None (default)'
@@ -661,7 +671,7 @@ def from_pyproject(
             else None
         )
 
-        return cls(
+        self = cls(
             name=name,
             version=version,
             description=description,
@@ -683,6 +693,8 @@ def from_pyproject(
             dynamic_metadata=dynamic_metadata or [],
             metadata_version=metadata_version,
         )
+        self._locked_metadata = True
+        return self
 
     def as_rfc822(self) -> RFC822Message:
         message = RFC822Message()

tests/test_standard_metadata.py

@@ -1066,6 +1066,25 @@ def test_version_dynamic() -> None:
     metadata.version = packaging.version.Version('1.2.3')
 
 
+def test_modify_dynamic() -> None:
+    metadata = pyproject_metadata.StandardMetadata.from_pyproject(
+        {
+            'project': {
+                'name': 'example',
+                'version': '1.2.3',
+                'dynamic': [
+                    'requires-python',
+                ],
+            },
+        }
+    )
+    metadata.requires_python = packaging.specifiers.SpecifierSet('>=3.12')
+    with pytest.raises(
+        AttributeError, match=re.escape('Field "version" is not dynamic')
+    ):
+        metadata.version = packaging.version.Version('1.2.3')
+
+
 def test_missing_keys_warns() -> None:
     with pytest.warns(
         pyproject_metadata.ConfigurationWarning,