Populate Python API error messages during deserialization

CHANGELOG.md

@@ -101,6 +101,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- Python client: Populate `APIError.message` while deserializing a property marked with `x-ms-primary-error-message`. [#7542](https://github.com/microsoft/kiota/issues/7542)
 - C#, Java, Go, PHP, Dart, TypeScript, Python and Ruby client: default value initialization in model classes for DateTime/Date/Time/UUID properties did not compile [#7404](https://github.com/microsoft/kiota/issues/7404)
 - All languages: default value initialization in model classes for numeric/boolean properties was missing [#7404](https://github.com/microsoft/kiota/issues/7404)
 - Fixed a bug where required query parameters from one HTTP operation were leaking into the path-item-level URL template, making them appear required for sibling operations on the same path. [#7292](https://github.com/microsoft/kiota/issues/7292)

src/Kiota.Builder/Writers/Python/CodeMethodWriter.cs

@@ -568,13 +568,30 @@ private void WriteDeserializerBodyForIntersectionModel(CodeClass parentClass, La
     private void WriteDeserializerBodyForInheritedModel(bool inherits, CodeMethod codeElement, CodeClass parentClass, LanguageWriter writer)
     {
         _codeUsingWriter.WriteInternalImports(parentClass, writer);
+        var customProperties = parentClass
+                                .GetPropertiesOfKind(CodePropertyKind.Custom)
+                                .Where(static x => !x.ExistsInBaseType)
+                                .OrderBy(static x => x.Name)
+                                .ToArray();
+        if (parentClass.IsErrorDefinition)
+        {
+            foreach (var primaryErrorMessageProperty in customProperties.Where(static x => x.IsPrimaryErrorMessage))
+            {
+                var deserializerName = $"deserialize_{primaryErrorMessageProperty.Name.CleanupSymbolName()}";
+                writer.StartBlock($"def {deserializerName}(n: ParseNode) -> None:");
+                writer.WriteLine($"value = n.{GetDeserializationMethodName(primaryErrorMessageProperty.Type, codeElement, parentClass)}");
+                writer.WriteLine($"setattr(self, '{primaryErrorMessageProperty.Name.SanitizeSingleQuote()}', value)");
+                writer.WriteLine("self.message = '' if value is None else str(value)");
+                writer.CloseBlock(string.Empty);
+            }
+        }
         writer.StartBlock($"fields: dict[str, Callable[[Any], {NoneKeyword}]] = {{");
-        foreach (var otherProp in parentClass
-                                        .GetPropertiesOfKind(CodePropertyKind.Custom)
-                                        .Where(static x => !x.ExistsInBaseType)
-                                        .OrderBy(static x => x.Name))
+        foreach (var otherProp in customProperties)
         {
-            writer.WriteLine($"\"{otherProp.WireName.SanitizeDoubleQuote()}\": lambda n : setattr(self, '{otherProp.Name}', n.{GetDeserializationMethodName(otherProp.Type, codeElement, parentClass)}),");
+            var deserializer = parentClass.IsErrorDefinition && otherProp.IsPrimaryErrorMessage ?
+                $"deserialize_{otherProp.Name.CleanupSymbolName()}" :
+                $"lambda n : setattr(self, '{otherProp.Name.SanitizeSingleQuote()}', n.{GetDeserializationMethodName(otherProp.Type, codeElement, parentClass)})";
+            writer.WriteLine($"\"{otherProp.WireName.SanitizeDoubleQuote()}\": {deserializer},");
         }
         writer.CloseBlock();
         if (inherits)

tests/Kiota.Builder.Tests/Writers/Python/CodeMethodWriterTests.cs

@@ -908,6 +908,61 @@ public void WritesDeSerializerBody()
         Assert.Contains("defined_in_parent", result, StringComparison.OrdinalIgnoreCase);
     }
     [Fact]
+    public void WritesPrimaryErrorMessageDeserializer()
+    {
+        setup();
+        parentClass.IsErrorDefinition = true;
+        parentClass.AddProperty(new CodeProperty
+        {
+            Name = "detail",
+            Kind = CodePropertyKind.Custom,
+            IsPrimaryErrorMessage = true,
+            Type = new CodeType
+            {
+                Name = "string"
+            }
+        });
+        method.Kind = CodeMethodKind.Deserializer;
+        method.IsAsync = false;
+
+        writer.Write(method);
+        var result = tw.ToString();
+
+        Assert.Contains("def deserialize_detail(n: ParseNode) -> None:", result);
+        Assert.Contains("value = n.get_str_value()", result);
+        Assert.Contains("setattr(self, 'detail', value)", result);
+        Assert.Contains("self.message = '' if value is None else str(value)", result);
+        Assert.Contains("\"detail\": deserialize_detail,", result);
+    }
+    [Fact]
+    public void EscapesPrimaryErrorMessagePropertyName()
+    {
+        setup();
+        parentClass.IsErrorDefinition = true;
+        parentClass.AddProperty(new CodeProperty
+        {
+            Name = "line1'\nline2",
+            SerializationName = "detail",
+            Kind = CodePropertyKind.Custom,
+            IsPrimaryErrorMessage = true,
+            Type = new CodeType
+            {
+                Name = "string"
+            }
+        });
+        method.Kind = CodeMethodKind.Deserializer;
+        method.IsAsync = false;
+
+        writer.Write(method);
+        var result = tw.ToString();
+
+        Assert.Contains("def deserialize_line1Line2(n: ParseNode) -> None:", result);
+        Assert.Contains("value = n.get_str_value()", result);
+        Assert.Contains("setattr(self, 'line1\\'\\nline2', value)", result);
+        Assert.Contains("self.message = '' if value is None else str(value)", result);
+        Assert.Contains("\"detail\": deserialize_line1Line2,", result);
+    }
+    [Fact]
     public void WritesInheritedSerializerBody()
     {
         setup(true);
@@ -1031,6 +1086,28 @@ public void EscapesWireNamesInSerializerAndDeserializerBody()
         Assert.Contains("\"line1\\\"\\nline2\": lambda n : setattr(self, 'dummy_string', n.get_str_value())", deserializerResult);
     }
     [Fact]
+    public void EscapesPropertyNamesInDeserializerBody()
+    {
+        setup();
+        parentClass.AddProperty(new CodeProperty
+        {
+            Name = "line1'\nline2",
+            SerializationName = "value",
+            Kind = CodePropertyKind.Custom,
+            Type = new CodeType
+            {
+                Name = "string"
+            }
+        });
+        method.Kind = CodeMethodKind.Deserializer;
+        method.IsAsync = false;
+
+        writer.Write(method);
+        var result = tw.ToString();
+
+        Assert.Contains("setattr(self, 'line1\\'\\nline2', n.get_str_value())", result);
+    }
+    [Fact]
     public void WritesMethodAsyncDescription()
     {
         setup();