ImportString fails to generate JsonSchema - in fastapi openapi docs generation

[amitschang]

Initial Checks

• I confirm that I'm using Pydantic V2

Description

We use pydantic with FastApi and have a model which contains an ImportString field associated with an endpoint and found that the openapi docs generation failed due to ImportString. Underlying issue is that pydantic could not create JsonSchema for the object, the error looks like:

 raise PydanticInvalidForJsonSchema(f'Cannot generate a JsonSchema for {error_info}')
pydantic.errors.PydanticInvalidForJsonSchema: Cannot generate a JsonSchema for core_schema.PlainValidatorFunctionSchema ({'type': 'no-info', 'function': <function import_string at 0x105a2c360>})

A workaround was to create a type-annotated version:

ImportString = Annotated[
    pydantic.ImportString, pydantic.WithJsonSchema({'type': 'string', 'description': 'fully qualified class name'})
]

I'm not sure if this is intended, but the docs don't seem to explicitly mention the json schema serialization.

Example Code

# Issue was in FastApi docs generation, but the below produces an error that 
# looks the same
import pydantic

pydantic.TypeAdapter(pydantic.ImportString).json_schema()

Python, Pydantic & OS Version

python -c "import pydantic.version; print(pydantic.version.version_info())"
             pydantic version: 2.7.1
        pydantic-core version: 2.18.2
          pydantic-core build: profile=release pgo=true
                 install path: /Users/arik/.miniconda3/lib/python3.11/site-packages/pydantic
               python version: 3.11.5 (main, Sep 11 2023, 08:31:25) [Clang 14.0.6 ]
                     platform: macOS-13.5-arm64-arm-64bit
             related packages: typing_extensions-4.9.0 fastapi-0.108.0
                       commit: unknown

[sydney-runkle]

@amitschang,

Ah, nice catch. We'll want to add json schema support to the ImportString type. PR welcome!

[amitschang]

@sydney-runkle, thanks for looking! I'll see if I have some time to work on such a PR

[amitschang]

@sydney-runkle , I made a PR - I can't seem to add the necessary labels to it (ready for review and relnotes-fix), but it is there