Populate Python API error messages during deserialization

[AayushP123]

Summary

Fixes #7542.

Populates the inherited Python APIError.message field when a generated error property is marked with x-ms-primary-error-message. The property itself was already deserialized, but the exception message remained unset.

Changes

• Emit a named deserializer for primary error message properties.
• Assign the parsed value to the generated property and populate APIError.message with an explicit None check and string coercion.
• Escape generated property-name literals used by setattr.
• Add a focused Python writer regression test and changelog entry.

Testing

• Generated the issue's ProblemDetails model and verified the output with python3 -m compileall.
• dotnet format whitespace kiota.slnx --no-restore --verify-no-changes --include src/Kiota.Builder/Writers/Python/CodeMethodWriter.cs tests/Kiota.Builder.Tests/Writers/Python/CodeMethodWriterTests.cs
• Focused writer tests (2 passed)
• Non-network builder suite (2,185 passed, 2 skipped); the two anonymous GitHub search tests return HTTP 403 in this environment.

[Copilot]

🟡 Not ready to approve

The generated assignment self.message = self.<primary> can propagate None into the exception message; it should coerce None to '' (and update the regression test accordingly) to match existing Python error-message semantics and TypeScript’s ?? "".

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Pull request overview

This PR updates the Python code generator so that when a model is an error type and one of its deserialized properties is marked with x-ms-primary-error-message, the generated deserialization logic also populates the inherited APIError.message field—matching behavior already seen in other languages and fixing #7542.

Changes:

• Emit a named per-field deserializer function for primary error message properties in Python model get_field_deserializers.
• In that named deserializer, assign the parsed value to both the generated property and APIError.message.
• Add a focused Python writer regression test and a changelog entry documenting the fix.

File summaries

File	Description
src/Kiota.Builder/Writers/Python/CodeMethodWriter.cs	Generates a named deserializer for primary error message properties and wires it into the deserializer map, while setting self.message.
tests/Kiota.Builder.Tests/Writers/Python/CodeMethodWriterTests.cs	Adds a regression test asserting the new named deserializer and the self.message assignment are emitted.
CHANGELOG.md	Documents the Python client behavior change for APIError.message population.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 2
• Review effort level: Lite

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[Copilot]

🟡 Not ready to approve

The new Python writer logic has a correctness issue in how it coerces/assigns APIError.message and also needs escaping when emitting schema-derived property names into single-quoted literals.

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Review details

Suppressed comments (2)

src/Kiota.Builder/Writers/Python/CodeMethodWriter.cs:582

• APIError.message is assigned using truthiness (or ''), which can silently drop valid falsy values (e.g., 0) and can also assign non-string types if the schema mistakenly marks a non-string property as x-ms-primary-error-message. Using an explicit None check and str(...) keeps the message consistently a string while still coercing None to an empty string.

                writer.StartBlock($"def deserialize_{primaryErrorMessageProperty.Name}(n: ParseNode) -> None:");
                writer.WriteLine($"self.{primaryErrorMessageProperty.Name} = n.{GetDeserializationMethodName(primaryErrorMessageProperty.Type, codeElement, parentClass)}");
                writer.WriteLine($"self.message = self.{primaryErrorMessageProperty.Name} or ''");

src/Kiota.Builder/Writers/Python/CodeMethodWriter.cs:592

• The property name is written into a single-quoted Python string literal for setattr(...) without escaping. Even though CodeProperty.Name is schema-derived via conventions, escaping at the emission site avoids literal injection/broken output if a name ever contains ', backslashes, or control characters.

            var deserializer = parentClass.IsErrorDefinition && otherProp.IsPrimaryErrorMessage ?
                $"deserialize_{otherProp.Name}" :
                $"lambda n : setattr(self, '{otherProp.Name}', n.{GetDeserializationMethodName(otherProp.Type, codeElement, parentClass)})";
            writer.WriteLine($"\"{otherProp.WireName.SanitizeDoubleQuote()}\": {deserializer},");

• Files reviewed: 3/3 changed files
• Comments generated: 0 new
• Review effort level: Lite

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[Copilot]

🟡 Not ready to approve

The Python writer currently interpolates schema-derived property names into Python identifier/attribute contexts for the named deserializer path without robust hardening, which can produce invalid code and presents a generated-source injection risk.

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Review details

Suppressed comments (1)

tests/Kiota.Builder.Tests/Writers/Python/CodeMethodWriterTests.cs:915

• The new primary-error-message deserializer test only covers a simple identifier name (detail). Since the writer now emits a named deserializer, it would be good to add a regression assertion using a hostile/irregular property name (e.g., containing ' and \n) to ensure the named-deserializer path is also safe/escaped (similar to the existing EscapesPropertyNamesInDeserializerBody coverage for the lambda path).

    public void WritesPrimaryErrorMessageDeserializer()
    {
        setup();
        parentClass.IsErrorDefinition = true;
        parentClass.AddProperty(new CodeProperty

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Lite

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