Decode cache data to the correct endpoint type

Use the return annotation to decode cached data to the correct type.
This follows the same logic FastAPI uses to JSON request bodies.

For the PickleCoder, this is a no-op as pickle already stores type
information in the serialised data.

fastapi_cache/coder.py

@@ -3,13 +3,17 @@ import datetime
 import json
 import pickle  # nosec:B403
 from decimal import Decimal
-from typing import Any, Callable
+from typing import Any, Callable, TypeVar, overload
 
 import pendulum
 from fastapi.encoders import jsonable_encoder
+from pydantic import BaseConfig, ValidationError, fields
 from starlette.responses import JSONResponse
 from starlette.templating import _TemplateResponse as TemplateResponse
 
+_T = TypeVar("_T")
+
+
 CONVERTERS: dict[str, Callable[[str], Any]] = {
     "date": lambda x: pendulum.parse(x, exact=True),
     "datetime": lambda x: pendulum.parse(x, exact=True),
@@ -49,6 +53,35 @@ class Coder:
     def decode(cls, value: str) -> Any:
         raise NotImplementedError
 
+    @overload
+    @classmethod
+    def decode_as_type(cls, value: str, type_: _T) -> _T:
+        ...
+
+    @overload
+    @classmethod
+    def decode_as_type(cls, value: str, *, type_: None) -> Any:
+        ...
+
+    @classmethod
+    def decode_as_type(cls, value: str, *, type_: _T | None) -> _T | Any:
+        """Decode value to the specific given type
+
+        The default implementation uses the Pydantic model system to convert the value.
+
+        """
+        result = cls.decode(value)
+        if type_ is not None:
+            field = fields.ModelField(
+                name="body", type_=type_, class_validators=None, model_config=BaseConfig
+            )
+            result, errors = field.validate(result, {}, loc=())
+            if errors is not None:
+                if not isinstance(errors, list):
+                    errors = [errors]
+                raise ValidationError(errors, type_)
+        return result
+
 
 class JsonCoder(Coder):
     @classmethod
@@ -72,3 +105,10 @@ class PickleCoder(Coder):
     @classmethod
     def decode(cls, value: str) -> Any:
         return pickle.loads(codecs.decode(value.encode(), "base64"))  # nosec:B403,B301
+
+    @classmethod
+    def decode_as_type(cls, value: str, *, type_: Any) -> Any:
+        # Pickle already produces the correct type on decoding, no point
+        # in paying an extra performance penalty for pydantic to discover
+        # the same.
+        return cls.decode(value)

fastapi_cache/decorator.py

@@ -10,6 +10,7 @@ else:
     from typing_extensions import ParamSpec
 
 from fastapi.concurrency import run_in_threadpool
+from fastapi.dependencies.utils import get_typed_return_annotation
 from starlette.requests import Request
 from starlette.responses import Response
 
@@ -79,6 +80,7 @@ def cache(
             (param for param in signature.parameters.values() if param.annotation is Response),
             None,
         )
+        return_type = get_typed_return_annotation(func)
 
         @wraps(func)
         async def inner(*args: P.args, **kwargs: P.kwargs) -> R:
@@ -139,7 +141,7 @@ def cache(
                 ttl, ret = 0, None
             if not request:
                 if ret is not None:
-                    return coder.decode(ret)
+                    return coder.decode_as_type(ret, type_=return_type)
                 ret = await ensure_async_func(*args, **kwargs)
                 try:
                     await backend.set(cache_key, coder.encode(ret), expire)
@@ -161,7 +163,7 @@ def cache(
                         response.status_code = 304
                         return response
                     response.headers["ETag"] = etag
-                return coder.decode(ret)
+                return coder.decode_as_type(ret, type_=return_type)
 
             ret = await ensure_async_func(*args, **kwargs)
             encoded_ret = coder.encode(ret)