Helpful Pydantic classes that I use for FastAPI apps.

models.py

import uuid
from datetime import datetime
from typing import Optional

from humps import camelize  # This is from the pyhumps library (https://pypi.org/project/pyhumps/)
from pydantic import BaseModel as PydanticBase


class BaseModel(PydanticBase):
    # Standard config settings.
    class Config:
        # Camelize will give us camelCase field names for external usage (docs, client-side app, etc.)
        # while maintaining snake_case for internal usage.
        alias_generator = camelize

        # Allow internal usage of snake_case field names.
        allow_population_by_field_name = True

        # Extra whitespace? Gross.
        anystr_strip_whitespace = True

        # If not a standard or Pydantic type, just check `isinstance`.
        arbitrary_types_allowed = True

        # Allow FastAPI to auto-convert database objects to Pydantic objects.
        orm_mode = True

        # Private fields are cool.
        underscore_attrs_are_private = True

        # Just makes enum comparisons a bit cleaner.
        # Allows for `if x == MyEnum.OPTION_A` instead of `if x == MyEnum.OPTION_A.value`.
        use_enum_values = True


class SavedBaseModel(BaseModel):
    # Every object from the database/cache/data-persistence-layer has these attributes.
    id: uuid.UUID
    created: datetime
    updated: datetime


class BaseIdentifier(BaseModel):
    # Potentially other ways to identify objects, but the UUID is standard.
    id: Optional[uuid.UUID] = None


class BaseFilter(BaseModel):
    page: int = 1
    per_page: int = 100

    # These properties make Django pagination easier, using slice notation on querysets.
    @property
    def start(self) -> int:
        return (self.page - 1) * self.per_page

    @property
    def end(self) -> int:
        return self.page * self.per_page

    # These properties make SQLAlchemy pagination easier.
    @property
    def offset(self) -> int:
        return (self.page - 1) * self.per_page

    @property
    def limit(self) -> int:
        return self.per_page