BUG: fix a bug where calling distutils.build_ext.finalize_options more than once would raise an exception

Author: neutrinoceros

newsfragments/5083.bugfix.rst

@@ -0,0 +1,2 @@
+Fixed a bug where calling ``build_ext.finalize_options`` more than once would
+raise an exception.

setuptools/_distutils/command/build_ext.py

@@ -276,10 +276,10 @@ def finalize_options(self) -> None:  # noqa: C901
         if self.undef:
             self.undef = self.undef.split(',')
 
-        if self.swig_opts is None:
-            self.swig_opts = []
-        else:
+        if isinstance(self.swig_opts, str):
             self.swig_opts = self.swig_opts.split(' ')
+        elif self.swig_opts is None:
+            self.swig_opts = []
 
         # Finally add the user include and library directories if requested
         if self.user:

setuptools/tests/test_build_ext.py

@@ -178,6 +178,17 @@ def C(file):
         assert example_stub.startswith(f"{build_lib}/mypkg/__pycache__/ext1")
         assert example_stub.endswith(".pyc")
 
+    def test_finalize_options_multiple_call(self):
+        """
+        Regression test. Check that calling build_ext.finalize_options
+        more than once doesn't raise an exception.
+        """
+        extension = Extension('spam.eggs', ['eggs.c'])
+        dist = Distribution(dict(ext_modules=[extension]))
+        cmd = build_ext(dist)
+        cmd.finalize_options()
+        cmd.finalize_options()
+
 
 class TestBuildExtInplace:
     def get_build_ext_cmd(self, optional: bool, **opts) -> build_ext: