fastapi - 💡(How to fix) Fix Broken optional orjson/ujson install can break importing fastapi.responses

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Example Code

import sys, importlib

sys.path.insert(0, "/path/to/fastapi")

real_import_module = importlib.import_module

def fake_import_module(name, package=None):
    if name == "ujson":
        raise ModuleNotFoundError("No module named 'ujson'")
    if name == "orjson":
        raise ImportError("simulated binary/load failure")
    return real_import_module(name, package)

importlib.import_module = fake_import_module

import fastapi.responses

Description

Recent fastapi.responses code catches ModuleNotFoundError when importing optional JSON response encoders:

try:
    ujson = cast(_UjsonModule, importlib.import_module("ujson"))
except ModuleNotFoundError:
    ujson = None

try:
    orjson = cast(_OrjsonModule, importlib.import_module("orjson"))
except ModuleNotFoundError:
    orjson = None

This handles the package-absent case, but it no longer handles the optional C-extension failure case where the module exists but cannot load its binary extension or a platform dependency. In that case, importlib.import_module("orjson") or importlib.import_module("ujson") can raise a plain ImportError, and importing fastapi.responses fails even if the user only needs JSONResponse, HTMLResponse, etc.

I noticed this while reviewing recent changes around free-threaded Python 3.14t support and the responses.py optional import cleanup.

Expected behavior: a broken optional orjson or ujson install should not break importing the general response module. It should behave like the package is unavailable until the deprecated optional response class is actually used.

Suggested fix: catch ImportError for these optional imports, as the previous direct imports did, or catch both ModuleNotFoundError and ImportError.

Suggested regression test: monkeypatch importlib.import_module to raise ImportError for orjson, then assert import fastapi.responses succeeds.

Operating System

Windows

Operating System Details

Windows 11, local repo checkout

FastAPI Version

Current master at ecace740

Python Version

Python 3.12.7

Additional Context

Related recent PR reviewed: #15149 (4b264878), "Support free-threaded Python 3.14t".