Fix optional response encoder import errors

fastapi/responses.py

@@ -24,16 +24,22 @@ class _OrjsonModule(Protocol):
     def dumps(self, __obj: Any, *, option: int = ...) -> bytes: ...
 
 
+ujson: _UjsonModule | None
+ujson_import_error: ImportError | None = None
 try:
     ujson = cast(_UjsonModule, importlib.import_module("ujson"))
-except ModuleNotFoundError:  # pragma: nocover
-    ujson = None  # type: ignore[assignment]
+except ImportError as e:
+    ujson = None
+    ujson_import_error = e
 
 
+orjson: _OrjsonModule | None
+orjson_import_error: ImportError | None = None
 try:
     orjson = cast(_OrjsonModule, importlib.import_module("orjson"))
-except ModuleNotFoundError:  # pragma: nocover
-    orjson = None  # type: ignore[assignment]
+except ImportError as e:
+    orjson = None
+    orjson_import_error = e
 
 
 @deprecated(
@@ -62,7 +68,10 @@ class UJSONResponse(JSONResponse):
     """
 
     def render(self, content: Any) -> bytes:
-        assert ujson is not None, "ujson must be installed to use UJSONResponse"
+        if ujson is None:
+            raise RuntimeError(
+                "ujson must be installed to use UJSONResponse"
+            ) from ujson_import_error
         return ujson.dumps(content, ensure_ascii=False).encode("utf-8")
 
 
@@ -92,7 +101,10 @@ class ORJSONResponse(JSONResponse):
     """
 
     def render(self, content: Any) -> bytes:
-        assert orjson is not None, "orjson must be installed to use ORJSONResponse"
+        if orjson is None:
+            raise RuntimeError(
+                "orjson must be installed to use ORJSONResponse"
+            ) from orjson_import_error
         return orjson.dumps(
             content, option=orjson.OPT_NON_STR_KEYS | orjson.OPT_SERIALIZE_NUMPY
         )

tests/test_deprecated_responses.py

@@ -1,5 +1,9 @@
+import importlib
+import sys
 import warnings
+from types import ModuleType
 
+import fastapi
 import pytest
 from fastapi import FastAPI
 from fastapi.exceptions import FastAPIDeprecationWarning
@@ -15,6 +19,33 @@ class Item(BaseModel):
     price: float
 
 
+def _import_responses_with_failed_optional_import(
+    monkeypatch: pytest.MonkeyPatch, module_name: str
+) -> ModuleType:
+    original_responses = sys.modules.pop("fastapi.responses", None)
+    had_responses_attr = hasattr(fastapi, "responses")
+    original_responses_attr = getattr(fastapi, "responses", None)
+    if had_responses_attr:
+        delattr(fastapi, "responses")
+
+    real_import_module = importlib.import_module
+
+    def import_module(name: str, package: str | None = None) -> ModuleType:
+        if name == module_name:
+            raise ImportError(f"broken {module_name}")
+        return real_import_module(name, package)
+
+    monkeypatch.setattr(importlib, "import_module", import_module)
+    try:
+        return real_import_module("fastapi.responses")
+    finally:
+        sys.modules.pop("fastapi.responses", None)
+        if original_responses is not None:
+            sys.modules["fastapi.responses"] = original_responses
+        if had_responses_attr:
+            fastapi.responses = original_responses_attr
+
+
 # ORJSON
 
 
@@ -47,6 +78,22 @@ def test_orjson_response_emits_deprecation_warning():
         ORJSONResponse(content={"hello": "world"})
 
 
+def test_orjson_import_error_does_not_break_responses_import(
+    monkeypatch: pytest.MonkeyPatch,
+):
+    responses = _import_responses_with_failed_optional_import(monkeypatch, "orjson")
+
+    response = responses.JSONResponse(content={"hello": "world"})
+    assert response.body == b'{"hello":"world"}'
+
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore", FastAPIDeprecationWarning)
+        with pytest.raises(RuntimeError, match="orjson must be installed") as exc_info:
+            responses.ORJSONResponse(content={"hello": "world"})
+    assert isinstance(exc_info.value.__cause__, ImportError)
+    assert str(exc_info.value.__cause__) == "broken orjson"
+
+
 # UJSON
 
 
@@ -77,3 +124,19 @@ def test_ujson_response_returns_correct_data():
 def test_ujson_response_emits_deprecation_warning():
     with pytest.warns(FastAPIDeprecationWarning, match="UJSONResponse is deprecated"):
         UJSONResponse(content={"hello": "world"})
+
+
+def test_ujson_import_error_does_not_break_responses_import(
+    monkeypatch: pytest.MonkeyPatch,
+):
+    responses = _import_responses_with_failed_optional_import(monkeypatch, "ujson")
+
+    response = responses.JSONResponse(content={"hello": "world"})
+    assert response.body == b'{"hello":"world"}'
+
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore", FastAPIDeprecationWarning)
+        with pytest.raises(RuntimeError, match="ujson must be installed") as exc_info:
+            responses.UJSONResponse(content={"hello": "world"})
+    assert isinstance(exc_info.value.__cause__, ImportError)
+    assert str(exc_info.value.__cause__) == "broken ujson"