Quick example using annotated types for Pint/Pydantic v2 compatibility

temperature.py

from openff.units import Quantity
from typing import Annotated
from pydantic import (
    BeforeValidator,
    AfterValidator,
    BaseModel,
    ValidationError,
    PositiveFloat,
)
from functools import partial


def to_quantity(quantity: Quantity | str) -> Quantity:
    try:
        return Quantity(quantity)
    except Exception as error:
        raise ValueError from error


def has_compatible_dimensionality(quantity: Quantity, unit: str) -> Quantity:
    if quantity.is_compatible_with(unit):
        return quantity
    else:
        raise ValueError(
            f"Dimensionality must be compatible with 'unit'",
        )


is_compatible_with_length = partial(has_compatible_dimensionality, unit="angstrom")
is_compatible_with_mass = partial(has_compatible_dimensionality, unit="dalton")
is_compatible_with_temperature = partial(has_compatible_dimensionality, unit="kelvin")
is_compatible_with_pressure = partial(has_compatible_dimensionality, unit="atm")

LengthQuantity = Annotated[
    Quantity,
    BeforeValidator(to_quantity),
    AfterValidator(is_compatible_with_length),
]

MassQuantity = Annotated[
    Quantity,
    BeforeValidator(to_quantity),
    AfterValidator(is_compatible_with_mass),
]

TemperatureQuantity = Annotated[
    Quantity,
    BeforeValidator(to_quantity),
    AfterValidator(is_compatible_with_temperature),
]

PressureQuantity = Annotated[
    Quantity,
    BeforeValidator(to_quantity),
    AfterValidator(is_compatible_with_pressure),
]

"""
Downstream code would something like
"""

# from openff.somewhere import TemperatureQuantity, PressureQuantity
from pydantic import BaseModel, PositiveFloat, Field


class DownstreamBaseModel(BaseModel):
    class Config:
        arbitrary_types_allowed = True
        # other stuff ...


class ThermoSettings(DownstreamBaseModel):
    temperature: TemperatureQuantity = Field(description="Simulation temperature")
    pressure: PressureQuantity = Field(description="Simulation pressure")
    ph: PositiveFloat | None = Field(None, description="Simulation pH")
    redox_potential: float | None = Field(
        None, description="Simulation redox potential"
    )


print(
    ThermoSettings(
        temperature=Quantity("300 K"), pressure=Quantity("1 atm")
    ).model_dump()
)
# {'temperature': <Quantity(300, 'kelvin')>, 'pressure': <Quantity(1, 'standard_atmosphere')>, 'ph': None, 'redox_potential': None}