Fix Python PATCH serialization for unset default fields

Author: swapneswarsundarray

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PythonClientCodegen.java

@@ -25,6 +25,8 @@
 import org.openapitools.codegen.meta.GeneratorMetadata;
 import org.openapitools.codegen.meta.Stability;
 import org.openapitools.codegen.meta.features.*;
+import org.openapitools.codegen.model.ModelMap;
+import org.openapitools.codegen.model.ModelsMap;
 import org.openapitools.codegen.utils.ModelUtils;
 import org.openapitools.codegen.utils.ProcessUtils;
 import org.slf4j.Logger;
@@ -434,6 +436,18 @@ public String modelTestFileFolder() {
         return outputFolder + File.separatorChar + testFolder;
     }
 
+    @Override
+    public ModelsMap postProcessModels(ModelsMap objs) {
+        // Process models to detect PATCH models (models starting with 'Patched')
+        for (ModelMap mo : objs.getModels()) {
+            CodegenModel cm = mo.getModel();
+            if (cm.getClassname().startsWith("Patched")) {
+                cm.vendorExtensions.put("isPatchedModel", true);
+            }
+        }
+        return super.postProcessModels(objs);
+    }
+
     public String packagePath() {
         return packageName.replace('.', File.separatorChar);
     }

modules/openapi-generator/src/main/resources/python/model_generic.mustache

@@ -164,11 +164,22 @@ class {{classname}}({{#parent}}{{{.}}}{{/parent}}{{^parent}}BaseModel{{/parent}}
             {{/isAdditionalPropertiesTrue}}
         ])
 
+        {{#classname}}
+        {{#isPatchedModel}}
+        _dict = self.model_dump(
+            by_alias=True,
+            exclude=excluded_fields,
+            exclude_unset=True,  # For PATCH models, exclude unset fields to avoid sending default values
+        )
+        {{/isPatchedModel}}
+        {{^isPatchedModel}}
         _dict = self.model_dump(
             by_alias=True,
             exclude=excluded_fields,
             exclude_none=True,
         )
+        {{/isPatchedModel}}
+{{/classname}}
         {{#allVars}}
         {{#isContainer}}
         {{#isArray}}