feat(python): add is_* and as_* casting methods for discriminated unions

[devin-ai-integration]

Description

Refs https://buildwithfern.slack.com/archives/C09NRQZ4A2X/p1763684501766159

Adds type-safe casting methods for discriminated union types in the Python SDK generator, matching the pattern used in the PHP generator. For each union variant, the generator now creates:

• is_*() methods to check if the union is of a specific variant type
• as_*() methods to cast to the specific type with runtime validation

Changes Made

• Added _add_is_method() to generate boolean type-checking methods for each union variant
• Added _add_as_method() to generate typed casting methods with runtime validation
• Both methods support Pydantic v1/v2 compatibility using the existing add_statements_v1_v2_or_both pattern
• Added version entry 4.39.0 in generators/python/sdk/versions.yml
• Methods are generated in discriminated_union_with_utils_generator.py for all discriminated union types

Example Generated Code:

animal: Animal = Animal.factory.dog(Dog(name="Fido"))

if animal.is_dog():
    dog = animal.as_dog()  # Returns Dog type
    print(dog.name)

# Raises ValueError if wrong type
cat = animal.as_cat()  # ValueError: Expected cat; got dog

Testing

• All CI checks passed (compile, lint, fastapi, python-sdk, pydantic seed tests)
• Verified Pydantic v1/v2 compatibility through seed tests
• Pre-commit hooks passed (ruff format, mypy)

Review Focus Areas

Critical Items:

1. AST Node Rendering: The implementation uses writer.write_node() for AST.Expression objects. Verify this pattern is correct and doesn't cause syntax errors in edge cases.
2. Pydantic v1/v2 Dual Paths: Check that both self.__root__ (v1) and self.root (v2) paths work correctly in the generated code.
3. Object Reconstruction: The as_*() methods reconstruct typed objects using dict(exclude_unset=True, exclude={discriminant}). Verify this correctly handles all union variant shapes (same_properties_as_object, single_property, no_properties).
4. No Properties Case: When a union variant has no properties, as_*() returns None. Confirm this is the expected behavior.

Implementation Notes:

• The methods are added after the visit() method in the generation flow
• Return types are computed based on the union variant shape
• Error messages include the expected and actual discriminant values for debugging

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Devin Session: https://app.devin.ai/sessions/57d48c907d15446caf2c77e9700bb45d
Requested by: [email protected] (@aditya-arolkar-swe)

[devin-ai-integration]

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