fastapi

Conventions

• URL-based API versioning: Use URL path versioning (e.g., /v1/, /api/v2/). Example context from fastapi/applications.py (lines 214-220):

                  ```python
                  from fastapi import FastAPI
  
                  app = FastAPI(openapi_url="/api/v1/openapi.json")
                  ```
                  """
              ),
  

• Data class style: Pydantic for API + dataclasses for internal: Use Pydantic for API schemas (40) and dataclasses for internal DTOs (10). Good separation. Example context from fastapi/sse.py (lines 47-57):

      if v is not None and "\0" in v:
          raise ValueError("SSE 'id' must not contain null characters")
      return _check_single_line(v, "id")
  
  
  class ServerSentEvent(BaseModel):
      """Represents a single Server-Sent Event.
  
      When `yield`ed from a *path operation function* that uses
      `response_class=EventSourceResponse`, each `ServerSentEvent` is encoded
      into the [SSE wire format](https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream)
  

• Background jobs with FastAPI BackgroundTasks: Use FastAPI BackgroundTasks for background task processing. Example context from fastapi/background.py (lines 1-10):

  from collections.abc import Callable
  from typing import Annotated, Any
  
  from annotated_doc import Doc
  from starlette.background import BackgroundTasks as StarletteBackgroundTasks
  from typing_extensions import ParamSpec
  
  P = ParamSpec("P")
  
  

• Data classes: Pydantic models: Use Pydantic models for structured data. 62/80 structured classes use this pattern. Example context from fastapi/sse.py (lines 47-57):

      if v is not None and "\0" in v:
          raise ValueError("SSE 'id' must not contain null characters")
      return _check_single_line(v, "id")
  
  
  class ServerSentEvent(BaseModel):
      """Represents a single Server-Sent Event.
  
      When `yield`ed from a *path operation function* that uses
      `response_class=EventSourceResponse`, each `ServerSentEvent` is encoded
      into the [SSE wire format](https://html.spec.whatwg.org/multipage/server-sent-events.html#parsing-an-event-stream)
  

• lowercase constant naming: Name constants using lowercase style. Example context from fastapi/params.py (lines 18-22):

  
  class ParamTypes(Enum):
      query = "query"
      header = "header"
      path = "path"
  

• Enum usage: Enum: Use Python enums for categorical values. Found 4 enum class(es). Example context from fastapi/params.py (lines 14-24):

      Undefined,
  )
  from .datastructures import _Unset
  
  
  class ParamTypes(Enum):
      query = "query"
      header = "header"
      path = "path"
      cookie = "cookie"
  

• Custom decorator pattern: @deprecated: Use custom decorator @deprecated (4 usages). Also uses: @asynccontextmanager. Example context from fastapi/responses.py (lines 34-44):

      orjson = cast(_OrjsonModule, importlib.import_module("orjson"))
  except ModuleNotFoundError:  # pragma: nocover
      orjson = None  # type: ignore[assignment]
  
  
  @deprecated(
      "UJSONResponse is deprecated, FastAPI now serializes data directly to JSON "
      "bytes via Pydantic when a return type or response model is set, which is "
      "faster and doesn't need a custom response class. Read more in the FastAPI "
      "docs: https://fastapi.tiangolo.com/advanced/custom-response/#orjson-or-response-model "
      "and https://fastapi.tiangolo.com/tutorial/response-model/",
  

• Limited exception chaining: Preserve exception context: use raise X from Y or raise X from None. Example context from fastapi/encoders.py (lines 350-356):

              data = vars(obj)
          except Exception as e:
              errors.append(e)
              raise ValueError(errors) from e
      return jsonable_encoder(
          data,
          include=include,
  

• Mixed validation approaches: Validate inputs and parameters: Use multiple validation approaches: Pydantic validation, Manual validation (ValueError/TypeError), Decorator-based validation.. Example context from fastapi/encoders.py (lines 21-27):

  from annotated_doc import Doc
  from fastapi.exceptions import PydanticV1NotSupportedError
  from fastapi.types import IncEx
  from pydantic import BaseModel
  from pydantic.networks import AnyUrl, NameEmail
  from pydantic.types import SecretBytes, SecretStr
  from pydantic_core import PydanticUndefinedType