Issue #20299: Argument Clinic custom converters may now change the default

value of c_default and py_default with a class member.

Misc/NEWS

@@ -101,6 +101,9 @@ Tests
 Tools/Demos
 -----------
 
+- Issue #20299: Argument Clinic custom converters may now change the default
+  value of c_default and py_default with a class member.
+
 - Issue #20287: Argument Clinic's output is now configurable, allowing
   delaying its output or even redirecting it to a separate file.
 

Tools/clinic/clinic.py

@@ -1893,8 +1893,10 @@ class CConverter(metaclass=CConverterAutoRegister):
                     self.__class__.__name__, default, name, types_str))
             self.default = default
 
-        self.c_default = c_default
-        self.py_default = py_default
+        if c_default:
+            self.c_default = c_default
+        if py_default:
+            self.py_default = py_default
 
         if annotation != unspecified:
             fail("The 'annotation' parameter is not currently permitted.")