Bug: openapi schema generation fails for Union of/in msgspec.Struct models

[aedify-swi]

Description

Hello!

In the latest versions(s) (I think this originates from the changes regarding nested models in openapi generation) we cannot use Unions of msgspec.Structs anymore. Neither as direct return types for routes nor nested within return types.

The result is a 500 Error. The MCVE below raises 'types.UnionType' object has no attribute '__qualname__' internally. In our production app I get typing.Union is not a module, class, method, or function. instead.

Cheers

URL to code causing the issue

No response

MCVE

import msgspec
import uvicorn
from litestar import Litestar, get


class SubStructA(msgspec.Struct):
    a: int


class SubStructB(msgspec.Struct):
    a: int


class StructyStruct(msgspec.Struct):
    sub: SubStructA | SubStructB


@get("/subunion")
async def testSubUnion() -> StructyStruct:
    return StructyStruct(SubStructA(0))


@get("/union")
async def testUnion() -> SubStructA | SubStructB:
    return SubStructA(0)


app = Litestar(route_handlers=[test2]) # or test
uvicorn.run(app)

Steps to reproduce

Run the example and browse to `localhost:8000/schema`

Screenshots

No response

Logs

No response

Litestar Version

2.5.0

Platform

• Linux
• Mac
• Windows
• Other (Please specify in the description above)

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[peterschutt]

Thanks for report @aedify-swi, confirmed.

Guessing that you are using 3.8 or 3.9 in prod app with Union instead of |?

Issue is with our internal schema generation plugin for Struct types (actually, other plugin types are likely vulnerable to this also).

litestar/litestar/_openapi/schema_generation/plugins/struct.py

Lines 21 to 22 in cc45c11

	def is_plugin_supported_field(self, field_definition: FieldDefinition) -> bool:
	return field_definition.is_subclass_of(Struct)

FieldDefinition.is_subclass_of() will return True if all elements of the union are subclasses of Struct, however the plugin doesn't make any effort to handle unions, nor should it b/c the schema generation scaffolding above it manages that. So if it receives a union, irrespective of the member types, it should return False.

Changing that to:

    def is_plugin_supported_field(self, field_definition: FieldDefinition) -> bool:
        return not field_definition.is_union and field_definition.is_subclass_of(Struct)

seems to sort it:

[aedify-swi]

Thanks for your quick reply @peterschutt!

Guessing that you are using 3.8 or 3.9 in prod app with Union instead of |?

Nope. 3.12 and exclusively |s. Your fix works regardless for our project even though the error message is a little different. ;)

[peterschutt]

Nope. 3.12

Any chance you can provide a traceback for your prod issue please?

[aedify-swi]

Even better, I was able to reproduce it.

import msgspec
import uvicorn
from litestar import Litestar, get


class SubStructA(msgspec.Struct):
    a: int


class SubStructB(msgspec.Struct):
    a: int


@get("/union")
async def testOptionalUnion() -> SubStructA | SubStructB | None:
    return SubStructA(0)


app = Litestar(route_handlers=[testOptionalUnion])  # or test
uvicorn.run(app)

This raises typing.Union is not a module, class, method, or function. in litestar.utils.typing.get_type_hints_with_generics_resolved

[github-actions]

A fix for this issue has been released in v2.5.1