mizan-fastapi: more Pythonic and declarative

Reworked the MVP code along the lines Ryth flagged. Same behavior
(11/11 tests still pass), tighter idiom.

executor.py:
- Replaced FunctionResult / FunctionError dataclasses with a MizanError
  exception hierarchy (NotFound, BadRequest, ValidationFailed,
  Unauthorized, Forbidden, NotImplementedYet, InternalError). Each
  carries its own ErrorCode + HTTP status; the dispatcher path raises
  rather than returning sentinel objects.
- Auth check uses match/case for the requirement (True / 'staff' /
  'superuser' / callable / other) — single declarative dispatch instead
  of an if/elif chain.
- Broke up the single 80-line execute_function into focused helpers:
  _resolve_function, _enforce_auth, _validate_input, _serialize,
  _invalidation_target. The execute_function body now reads as five
  declarative steps.
- Input validation uses Pydantic's model_fields[name].is_required()
  directly and a list comprehension for required-field reporting,
  instead of round-tripping through model_json_schema().

router.py:
- POST /call/ now declares its body as a Pydantic CallBody model;
  FastAPI handles parsing + envelope validation. No more manual
  await request.json() + dict[get] dancing.
- Endpoint bodies shrink to 3-5 lines each. Context fetch uses a
  dict comprehension over the function group.
- mizan_exception_handler renders MizanError to the protocol's
  {error: {code, message, details}} envelope.
- mizan_validation_handler maps FastAPI's RequestValidationError to
  the same envelope under BAD_REQUEST so the wire format is uniform
  whether the failure is body-shape or business validation.

__init__.py: exposes the full exception hierarchy + both handlers
so consumers can wire them onto their FastAPI app declaratively:

    app.add_exception_handler(MizanError, mizan_exception_handler)
    app.add_exception_handler(RequestValidationError, mizan_validation_handler)

Verified: mizan-core 15/15, mizan-django 348 pass, mizan-fastapi 11/11.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backends/mizan-fastapi/src/mizan_fastapi/__init__.py

@@ -1,30 +1,54 @@
 """
 mizan-fastapi — FastAPI backend adapter for the Mizan protocol.
 
-Provides HTTP RPC dispatch and context bundling on top of mizan-core's
+HTTP RPC dispatch and context bundling on top of mizan-core's function
-function registry. Channels, Forms, Shapes, SSR are out of scope for
+registry. Channels, Forms, Shapes, SSR are out of scope — FastAPI
-the FastAPI adapter — FastAPI projects use native equivalents (WebSocket,
+projects use native equivalents (WebSocket, Pydantic, ORM-of-choice,
-Pydantic models, ORM-of-choice, server-side rendering frameworks).
+SSR frameworks).
 
 Usage:
     from fastapi import FastAPI
-    from mizan_fastapi import router as mizan_router
+    from mizan_fastapi import router, mizan_exception_handler, MizanError
 
     app = FastAPI()
-    app.include_router(mizan_router, prefix="/api/mizan")
+    app.include_router(router, prefix="/api/mizan")
+    app.add_exception_handler(MizanError, mizan_exception_handler)
 
     # Register your @client-decorated functions
     from mizan_core.client.function import client
-    from .my_functions import *  # noqa
+    from mizan_core.registry import register
+    from .my_functions import echo
+    register(echo, "echo")
 """
 
-from .router import router
+from .executor import (
-from .executor import execute_function, ErrorCode, FunctionError, FunctionResult
+    ErrorCode,
+    MizanError,
+    NotFound,
+    BadRequest,
+    ValidationFailed,
+    Unauthorized,
+    Forbidden,
+    NotImplementedYet,
+    InternalError,
+    compute_invalidation,
+    execute_function,
+)
+from .router import router, mizan_exception_handler, mizan_validation_handler
 
 __all__ = [
     "router",
+    "mizan_exception_handler",
+    "mizan_validation_handler",
     "execute_function",
+    "compute_invalidation",
     "ErrorCode",
-    "FunctionError",
+    "MizanError",
-    "FunctionResult",
+    "NotFound",
+    "BadRequest",
+    "ValidationFailed",
+    "Unauthorized",
+    "Forbidden",
+    "NotImplementedYet",
+    "InternalError",
 ]

backends/mizan-fastapi/src/mizan_fastapi/executor.py

@@ -1,17 +1,14 @@
 """
-RPC dispatch — looks up registered functions by name, validates input
+RPC dispatch — looks up registered functions, validates input against the
-against the function's Pydantic Input model, executes, and returns the
+function's Pydantic Input model, executes, and returns the serialized result.
-result wrapped in a normalized FunctionResult / FunctionError.
 
-Backend-agnostic where possible. The only FastAPI-specific bits are the
+Errors raise typed exceptions (MizanError subclasses). Wire those to JSON
-Request type hint (kept loose as Any) and the auth-check mechanism, which
+responses by registering `mizan_exception_handler` on the FastAPI app, or
-expects FastAPI to populate request.state.user via dependency injection
+let them propagate to your own handler.
-or middleware before dispatch.
 """
 
 from __future__ import annotations
 
-from dataclasses import dataclass
 from enum import Enum
 from typing import Any
 
@@ -20,7 +17,7 @@ from pydantic import BaseModel, ValidationError
 from mizan_core.registry import get_function
 
 
-# ─── Error / Result types ───────────────────────────────────────────────────
+# ─── Error taxonomy ─────────────────────────────────────────────────────────
 
 
 class ErrorCode(str, Enum):
@@ -33,164 +30,168 @@ class ErrorCode(str, Enum):
     INTERNAL_ERROR = "INTERNAL_ERROR"
 
 
-@dataclass
+_STATUS = {
-class FunctionError:
+    ErrorCode.NOT_FOUND: 404,
-    code: ErrorCode
+    ErrorCode.BAD_REQUEST: 400,
-    message: str
+    ErrorCode.VALIDATION_ERROR: 422,
-    details: dict[str, Any] | None = None
+    ErrorCode.UNAUTHORIZED: 401,
+    ErrorCode.FORBIDDEN: 403,
+    ErrorCode.NOT_IMPLEMENTED: 501,
+    ErrorCode.INTERNAL_ERROR: 500,
+}
 
 
-@dataclass
+class MizanError(Exception):
-class FunctionResult:
+    """Base for protocol-level dispatch errors."""
-    data: Any  # serialized return value (dict, list, primitive, or Pydantic model_dump)
+
+    code: ErrorCode = ErrorCode.INTERNAL_ERROR
+
+    def __init__(self, message: str, *, details: dict[str, Any] | None = None) -> None:
+        super().__init__(message)
+        self.message = message
+        self.details = details
+
+    @property
+    def status_code(self) -> int:
+        return _STATUS[self.code]
+
+
+class NotFound(MizanError):           code = ErrorCode.NOT_FOUND          # noqa: E701
+class BadRequest(MizanError):         code = ErrorCode.BAD_REQUEST        # noqa: E701
+class ValidationFailed(MizanError):   code = ErrorCode.VALIDATION_ERROR   # noqa: E701
+class Unauthorized(MizanError):       code = ErrorCode.UNAUTHORIZED       # noqa: E701
+class Forbidden(MizanError):          code = ErrorCode.FORBIDDEN          # noqa: E701
+class NotImplementedYet(MizanError):  code = ErrorCode.NOT_IMPLEMENTED    # noqa: E701
+class InternalError(MizanError):      code = ErrorCode.INTERNAL_ERROR     # noqa: E701
 
 
 # ─── Auth ───────────────────────────────────────────────────────────────────
 
 
-def _check_auth(request: Any, auth_requirement: Any) -> FunctionError | None:
+def _user(request: Any) -> Any:
-    """
+    return getattr(getattr(request, "state", None), "user", None)
-    Verify the request meets the function's auth requirement.
 
-    The auth value comes from @client(auth=...). FastAPI projects are expected
+
-    to populate `request.state.user` (or compatible) via middleware. If
+def _is_authenticated(user: Any) -> bool:
-    request has no `.state` or `.state.user`, treats the user as anonymous.
+    return bool(user) and getattr(user, "is_authenticated", True)
-    """
+
-    if auth_requirement is None:
+
+def _enforce_auth(request: Any, requirement: Any) -> None:
+    """Verify the request meets the function's @client(auth=...) requirement, or raise."""
+    if requirement is None:
+        return
+
+    user = _user(request)
+
+    match requirement:
+        case True:
+            if not _is_authenticated(user):
+                raise Unauthorized("Authentication required")
+        case "staff":
+            if not _is_authenticated(user):
+                raise Unauthorized("Authentication required")
+            if not getattr(user, "is_staff", False):
+                raise Forbidden("Staff access required")
+        case "superuser":
+            if not _is_authenticated(user):
+                raise Unauthorized("Authentication required")
+            if not getattr(user, "is_superuser", False):
+                raise Forbidden("Superuser access required")
+        case f if callable(f):
+            if not f(request):
+                raise Forbidden("Permission denied")
+        case other:
+            raise InternalError(f"Unknown auth requirement: {other!r}")
+
+
+# ─── Input validation ───────────────────────────────────────────────────────
+
+
+def _validate_input(input_cls: Any, input_data: Any) -> BaseModel | None:
+    """Validate input_data against the function's Input model. Returns the instance or None."""
+    if input_cls in (None, BaseModel) or not getattr(input_cls, "model_fields", None):
         return None
 
-    user = getattr(getattr(request, "state", None), "user", None)
+    fields = input_cls.model_fields
-    is_authenticated = bool(user) and getattr(user, "is_authenticated", True)
+    required = [name for name, f in fields.items() if f.is_required()]
 
-    if auth_requirement is True:
+    if not input_data:
-        if not is_authenticated:
+        if required:
-            return FunctionError(ErrorCode.UNAUTHORIZED, "Authentication required")
+            raise ValidationFailed(
-        return None
+                "Input validation failed",
+                details={"fields": {name: ["Field required"] for name in required}},
+            )
+        return input_cls()
 
-    if auth_requirement == "staff":
+    if not isinstance(input_data, dict):
-        if not is_authenticated:
+        raise BadRequest(f"Input must be an object, got {type(input_data).__name__}")
-            return FunctionError(ErrorCode.UNAUTHORIZED, "Authentication required")
-        if not getattr(user, "is_staff", False):
-            return FunctionError(ErrorCode.FORBIDDEN, "Staff access required")
-        return None
 
-    if auth_requirement == "superuser":
+    try:
-        if not is_authenticated:
+        return input_cls(**input_data)
-            return FunctionError(ErrorCode.UNAUTHORIZED, "Authentication required")
+    except ValidationError as e:
-        if not getattr(user, "is_superuser", False):
+        raise ValidationFailed(
-            return FunctionError(ErrorCode.FORBIDDEN, "Superuser access required")
+            "Input validation failed",
-        return None
+            details={"errors": e.errors()},
-
+        ) from e
-    if callable(auth_requirement):
-        if not auth_requirement(request):
-            return FunctionError(ErrorCode.FORBIDDEN, "Permission denied")
-        return None
-
-    return FunctionError(
-        ErrorCode.INTERNAL_ERROR,
-        f"Unknown auth requirement: {auth_requirement!r}",
-    )
 
 
 # ─── Dispatch ───────────────────────────────────────────────────────────────
 
 
+def _resolve_function(fn_name: str) -> Any:
+    view_class = get_function(fn_name)
+    if view_class is None:
+        raise NotFound("Function not found")
+    if getattr(view_class, "_meta", {}).get("private"):
+        raise Forbidden("Function is not client-callable")
+    return view_class
+
+
+def _serialize(result: Any) -> Any:
+    return result.model_dump(mode="json") if isinstance(result, BaseModel) else result
+
+
 def execute_function(
     request: Any,
     fn_name: str,
     input_data: dict[str, Any] | None = None,
-) -> FunctionResult | FunctionError:
+) -> Any:
-    """
+    """Dispatch a registered function. Returns the serialized result, or raises MizanError."""
-    Look up a registered function by name, validate input, execute, return result.
+    view_class = _resolve_function(fn_name)
-    """
+    _enforce_auth(request, view_class._meta.get("auth"))
-    view_class = get_function(fn_name)
-    if view_class is None:
-        return FunctionError(ErrorCode.NOT_FOUND, "Function not found")
-
-    meta = getattr(view_class, "_meta", {})
-
-    if meta.get("private"):
-        return FunctionError(ErrorCode.FORBIDDEN, "Function is not client-callable")
-
-    auth_error = _check_auth(request, meta.get("auth"))
-    if auth_error is not None:
-        return auth_error
 
     view = view_class(request)
-    input_cls = view.Input
+    validated = _validate_input(view.Input, input_data)
 
-    # Pydantic input validation
-    has_fields = bool(getattr(input_cls, "model_fields", None)) if input_cls else False
-
-    if input_data is not None and has_fields:
-        if not isinstance(input_data, dict):
-            return FunctionError(
-                ErrorCode.BAD_REQUEST,
-                f"Input must be an object, got {type(input_data).__name__}",
-            )
-        try:
-            validated_input = input_cls(**input_data)
-        except ValidationError as e:
-            return FunctionError(
-                ErrorCode.VALIDATION_ERROR,
-                "Input validation failed",
-                details={"errors": e.errors()},
-            )
-    elif has_fields:
-        # Function expects input but none provided
-        required = input_cls.model_json_schema().get("required", [])
-        if required:
-            return FunctionError(
-                ErrorCode.VALIDATION_ERROR,
-                "Input validation failed",
-                details={"fields": {field: ["Field required"] for field in required}},
-            )
-        validated_input = input_cls()
-    else:
-        validated_input = None
-
-    # Execute. The wrapper's call(input) always takes the input arg,
-    # passing None when the function has no fields.
     try:
-        result = view.call(validated_input)
+        result = view.call(validated)
     except NotImplementedError as e:
-        return FunctionError(ErrorCode.NOT_IMPLEMENTED, str(e) or "Not implemented")
+        raise NotImplementedYet(str(e) or "Not implemented") from e
+    except MizanError:
+        raise
     except Exception as e:
-        return FunctionError(ErrorCode.INTERNAL_ERROR, str(e))
+        raise InternalError(str(e)) from e
 
-    # Serialize Pydantic models to plain dicts
+    return _serialize(result)
-    if isinstance(result, BaseModel):
-        return FunctionResult(data=result.model_dump(mode="json"))
-    return FunctionResult(data=result)
 
 
 # ─── Invalidation ───────────────────────────────────────────────────────────
 
 
 def compute_invalidation(view_class: Any, input_data: dict[str, Any] | None) -> list[Any]:
-    """
+    """Build the `invalidate` list from @client(affects=...) metadata, auto-scoping when arg names match context params."""
-    Build the invalidate list for a mutation response from the function's
+    affects = getattr(view_class, "_meta", {}).get("affects") or []
-    @client(affects=...) metadata. Auto-scopes to params when the mutation's
+    return [_invalidation_target(target, input_data or {}) for target in affects]
-    arg names overlap with a context's params.
-    """
-    meta = getattr(view_class, "_meta", {})
-    affects = meta.get("affects")
-    if not affects:
-        return []
 
-    out: list[Any] = []
+
-    for target in affects:
+def _invalidation_target(target: dict[str, Any], input_data: dict[str, Any]) -> Any:
-        target_type = target.get("type")
+    match target.get("type"):
-        target_name = target.get("name")
+        case "context":
-        if target_type == "context":
+            name = target["name"]
-            scope_params = target.get("params") or {}
+            scope_keys = (target.get("params") or {}).keys()
-            if scope_params and input_data:
+            scoped = {k: input_data[k] for k in scope_keys if k in input_data}
-                # Auto-scope: include matching param values
+            return {"context": name, "params": scoped} if scoped else name
-                matched = {k: input_data[k] for k in scope_params if k in input_data}
+        case "function":
-                if matched:
+            return {"function": target["name"]}
-                    out.append({"context": target_name, "params": matched})
+        case _:
-                    continue
+            return target
-            out.append(target_name)
-        elif target_type == "function":
-            out.append({"function": target_name})
-    return out

backends/mizan-fastapi/src/mizan_fastapi/router.py

@@ -4,13 +4,12 @@ FastAPI router exposing Mizan's HTTP endpoints:
     POST /call/                — RPC dispatch
     GET  /ctx/{context_name}/  — bundled context fetch
 
-Mount under /api/mizan in your FastAPI app:
-
     from fastapi import FastAPI
-    from mizan_fastapi import router as mizan_router
+    from mizan_fastapi import router, mizan_exception_handler, MizanError
 
     app = FastAPI()
-    app.include_router(mizan_router, prefix="/api/mizan")
+    app.include_router(router, prefix="/api/mizan")
+    app.add_exception_handler(MizanError, mizan_exception_handler)
 """
 
 from __future__ import annotations
@@ -18,14 +17,16 @@ from __future__ import annotations
 from typing import Any
 
 from fastapi import APIRouter, Request
+from fastapi.exceptions import RequestValidationError
 from fastapi.responses import JSONResponse
+from pydantic import BaseModel, Field
 
 from mizan_core.registry import get_context_groups, get_function
 
 from .executor import (
     ErrorCode,
-    FunctionError,
+    MizanError,
-    FunctionResult,
+    NotFound,
     compute_invalidation,
     execute_function,
 )
@@ -34,85 +35,58 @@ from .executor import (
 router = APIRouter()
 
 
-_HTTP_STATUS_FOR_ERROR = {
+def _no_store(payload: Any, status_code: int = 200) -> JSONResponse:
-    ErrorCode.NOT_FOUND: 404,
+    return JSONResponse(payload, status_code=status_code, headers={"Cache-Control": "no-store"})
-    ErrorCode.BAD_REQUEST: 400,
-    ErrorCode.VALIDATION_ERROR: 422,
-    ErrorCode.UNAUTHORIZED: 401,
-    ErrorCode.FORBIDDEN: 403,
-    ErrorCode.NOT_IMPLEMENTED: 501,
-    ErrorCode.INTERNAL_ERROR: 500,
-}
 
 
-def _error_response(err: FunctionError) -> JSONResponse:
+# ─── Endpoints ──────────────────────────────────────────────────────────────
-    body: dict[str, Any] = {
+
-        "error": {
+
-            "code": err.code.value,
+class CallBody(BaseModel):
-            "message": err.message,
+    fn: str = Field(..., min_length=1)
-        }
+    args: dict[str, Any] = Field(default_factory=dict)
-    }
-    if err.details:
-        body["error"]["details"] = err.details
-    return JSONResponse(
-        body,
-        status_code=_HTTP_STATUS_FOR_ERROR.get(err.code, 500),
-        headers={"Cache-Control": "no-store"},
-    )
 
 
 @router.post("/call/")
-async def function_call(request: Request) -> JSONResponse:
+async def function_call(body: CallBody, request: Request) -> JSONResponse:
-    """RPC dispatch — `{"fn": "name", "args": {...}}` → `{"result": ..., "invalidate": [...]}`."""
+    """RPC dispatch — `{"fn": "...", "args": {...}}` → `{"result": ..., "invalidate": [...]}`."""
-    try:
+    result = execute_function(request, body.fn, body.args)
-        body = await request.json()
+    invalidate = compute_invalidation(get_function(body.fn), body.args)
-    except Exception:
+    return _no_store({"result": result, "invalidate": invalidate})
-        return _error_response(
-            FunctionError(ErrorCode.BAD_REQUEST, "Invalid JSON body")
-        )
-
-    if not isinstance(body, dict):
-        return _error_response(
-            FunctionError(ErrorCode.BAD_REQUEST, "Body must be an object")
-        )
-
-    fn_name = body.get("fn")
-    if not fn_name or not isinstance(fn_name, str):
-        return _error_response(
-            FunctionError(ErrorCode.BAD_REQUEST, "Missing or invalid 'fn' field")
-        )
-
-    args = body.get("args", {})
-
-    outcome = execute_function(request, fn_name, args)
-    if isinstance(outcome, FunctionError):
-        return _error_response(outcome)
-
-    view_class = get_function(fn_name)
-    invalidate = compute_invalidation(view_class, args)
-
-    return JSONResponse(
-        {"result": outcome.data, "invalidate": invalidate},
-        headers={"Cache-Control": "no-store"},
-    )
 
 
 @router.get("/ctx/{context_name}/")
 async def context_fetch(context_name: str, request: Request) -> JSONResponse:
-    """Bundled context fetch — returns `{function_name: result, ...}` for every function in the named context."""
+    """Bundled context fetch — `{function_name: result, ...}` for every function in the context."""
-    groups = get_context_groups()
+    fn_names = get_context_groups().get(context_name)
-    fn_names = groups.get(context_name)
     if not fn_names:
-        return _error_response(
+        raise NotFound(f"Context '{context_name}' not found")
-            FunctionError(ErrorCode.NOT_FOUND, f"Context '{context_name}' not found")
-        )
 
     params = dict(request.query_params)
-    result: dict[str, Any] = {}
+    bundled = {fn: execute_function(request, fn, params) for fn in fn_names}
-    for fn_name in fn_names:
+    return _no_store(bundled)
-        outcome = execute_function(request, fn_name, params)
-        if isinstance(outcome, FunctionError):
-            return _error_response(outcome)
-        result[fn_name] = outcome.data
 
-    return JSONResponse(result, headers={"Cache-Control": "no-store"})
+
+# ─── Exception handler ──────────────────────────────────────────────────────
+
+
+async def mizan_exception_handler(_request: Request, exc: MizanError) -> JSONResponse:
+    """FastAPI exception handler — renders MizanError to the protocol's error envelope."""
+    body: dict[str, Any] = {"error": {"code": exc.code.value, "message": exc.message}}
+    if exc.details:
+        body["error"]["details"] = exc.details
+    return _no_store(body, status_code=exc.status_code)
+
+
+async def mizan_validation_handler(_request: Request, exc: RequestValidationError) -> JSONResponse:
+    """Maps malformed request bodies (invalid JSON, missing top-level fields) to BAD_REQUEST."""
+    return _no_store(
+        {
+            "error": {
+                "code": ErrorCode.BAD_REQUEST.value,
+                "message": "Invalid request body",
+                "details": {"errors": exc.errors()},
+            }
+        },
+        status_code=400,
+    )

backends/mizan-fastapi/tests/test_dispatch.py

@@ -4,12 +4,18 @@ from __future__ import annotations
 
 import pytest
 from fastapi import FastAPI
+from fastapi.exceptions import RequestValidationError
 from fastapi.testclient import TestClient
 from pydantic import BaseModel
 
 from mizan_core.client.function import client
 from mizan_core.registry import clear_registry, register
-from mizan_fastapi import router as mizan_router
+from mizan_fastapi import (
+    MizanError,
+    mizan_exception_handler,
+    mizan_validation_handler,
+    router as mizan_router,
+)
 
 
 # ─── Fixtures ───────────────────────────────────────────────────────────────
@@ -61,6 +67,8 @@ def app():
 
     fastapi_app = FastAPI()
     fastapi_app.include_router(mizan_router, prefix="/api/mizan")
+    fastapi_app.add_exception_handler(MizanError, mizan_exception_handler)
+    fastapi_app.add_exception_handler(RequestValidationError, mizan_validation_handler)
 
     yield fastapi_app