I’m working to build a function parameter validation library using pydantic. We want to be able to validate parameters’ types and values. Types are easy enough, but I’m having a hard time creating a class to validate values. Specifically, the first class I want to build is one that requires the value to be in a user-defined interval.

So far, I’ve written a decorator and a function-based version of ValueInInterval. However, I would prefer to use a class-based approach. Here’s an MRE of my issue:

from typing import Any
from typing_extensions import Annotated
from pydantic import Field, validate_call


class ValueInInterval:
    def __init__(
        self,
        type_definition: Any,
        start: Any,
        end: Any,
        include_start: bool = True,
        include_end: bool = True,
    ):
        self.type_definition = type_definition
        self.start = start
        self.end = end
        self.include_start = include_start
        self.include_end = include_end

    def __call__(self):
        return Annotated[self.type_definition, self.create_field()]

    def create_field(self) -> Field:
        field_config = {}
        if self.include_start:
            field_config.update({"ge": self.start})
        else:
            field_config.update({"gt": self.start})
        if self.include_end:
            field_config.update({"le": self.end})
        else:
            field_config.update({"lt": self.end})

        return Field(**field_config)

    def __get_pydantic_core_schema__(
        self,
        handler,
    ):
        schema = handler(self.type_definition)

        if self.include_start:
            schema.update({"ge": self.start})
        else:
            schema.update({"gt": self.start})

        if self.include_end:
            schema.update({"le": self.end})
        else:
            schema.update({"lt": self.end})

        return schema


@validate_call()
def test_interval(
    value: ValueInInterval(type_definition=int, start=1, end=10),
):
    print(value)


test_interval(value=1)  # should succeed
test_interval(value=0)  # should fail

Running this code in the PyCharm’s Python console, I get the following error:

Traceback (most recent call last):
  File "C:Program FilesJetBrainsPyCharm 2023.2pluginspythonhelperspydevpydevconsole.py", line 364, in runcode
    coro = func()
  File "<input>", line 56, in <module>
  File "C:projectsdjango-postgres-loadervenvlibsite-packagespydanticvalidate_call_decorator.py", line 56, in validate
    validate_call_wrapper = _validate_call.ValidateCallWrapper(function, config, validate_return, local_ns)
  File "C:projectsdjango-postgres-loadervenvlibsite-packagespydantic_internal_validate_call.py", line 57, in __init__
    schema = gen_schema.clean_schema(gen_schema.generate_schema(function))
  File "C:projectsdjango-postgres-loadervenvlibsite-packagespydantic_internal_generate_schema.py", line 512, in generate_schema
    schema = self._generate_schema_inner(obj)
  File "C:projectsdjango-postgres-loadervenvlibsite-packagespydantic_internal_generate_schema.py", line 789, in _generate_schema_inner
    return self.match_type(obj)
  File "C:projectsdjango-postgres-loadervenvlibsite-packagespydantic_internal_generate_schema.py", line 856, in match_type
    return self._callable_schema(obj)
  File "C:projectsdjango-postgres-loadervenvlibsite-packagespydantic_internal_generate_schema.py", line 1692, in _callable_schema
    arg_schema = self._generate_parameter_schema(name, annotation, p.default, parameter_mode)
  File "C:projectsdjango-postgres-loadervenvlibsite-packagespydantic_internal_generate_schema.py", line 1414, in _generate_parameter_schema
    schema = self._apply_annotations(source_type, annotations)
  File "C:projectsdjango-postgres-loadervenvlibsite-packagespydantic_internal_generate_schema.py", line 1890, in _apply_annotations
    schema = get_inner_schema(source_type)
  File "C:projectsdjango-postgres-loadervenvlibsite-packagespydantic_internal_schema_generation_shared.py", line 83, in __call__
    schema = self._handler(source_type)
  File "C:projectsdjango-postgres-loadervenvlibsite-packagespydantic_internal_generate_schema.py", line 1869, in inner_handler
    from_property = self._generate_schema_from_property(obj, source_type)
  File "C:projectsdjango-postgres-loadervenvlibsite-packagespydantic_internal_generate_schema.py", line 677, in _generate_schema_from_property
    schema = get_schema(source)
  File "<input>", line 40, in __get_pydantic_core_schema__
TypeError: __call__() takes 1 positional argument but 2 were given

I strongly suspect that I’ve not written the __get_pydantic_core_schema__ correctly. Note that I’m using pydantic version 2.7 and Python 3.11.