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Example Code

pip install 'fastapi<0.124.0'

Description

After upgrading to FastAPI 0.124.0, our application fails to start with Python 3.12. The error occurs during route registration when FastAPI tries to evaluate type annotations.

Environment:

• FastAPI version: 0.124.0
• Python version: 3.12.12
• OS: macOS (Darwin 24.1.0) - also affects Windows
• Starlette version: 0.50.0

Error Traceback:

Traceback (most recent call last):
File "", line 198, in _run_module_as_main
File "", line 88, in _run_code
File "/path/to/site-packages/langflow/main.py", line 33, in
from langflow.main import setup_app
File "/path/to/site-packages/langflow/main.py", line 25, in
from langflow.api import health_check_router, log_router, router
File "/path/to/site-packages/langflow/api/init.py", line 1, in
from langflow.api.health_check_router import health_check_router
File "/path/to/site-packages/langflow/api/health_check_router.py", line 39, in
@health_check_router.get("/health_check")
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/path/to/site-packages/fastapi/routing.py", line 1128, in decorator
self.add_api_route(
File "/path/to/site-packages/fastapi/routing.py", line 1067, in add_api_route
route = route_class(
File "/path/to/site-packages/fastapi/routing.py", line 686, in init
self.dependant = get_dependant(
File "/path/to/site-packages/fastapi/dependencies/utils.py", line 303, in get_dependant
sub_dependant = get_dependant(
File "/path/to/site-packages/fastapi/dependencies/utils.py", line 277, in get_dependant
endpoint_signature = get_typed_signature(call)
File "/path/to/site-packages/fastapi/dependencies/utils.py", line 214, in get_typed_signature
signature = inspect.signature(call, eval_str=True)
File "/path/to/lib/python3.12/inspect.py", line 3348, in signature
return Signature.from_callable(obj, follow_wrapped=follow_wrapped,
File "/path/to/lib/python3.12/inspect.py", line 3085, in from_callable
return _signature_from_callable(obj, sigcls=cls,
File "/path/to/lib/python3.12/inspect.py", line 2597, in _signature_from_callable
return _signature_from_function(sigcls, obj,
File "/path/to/lib/python3.12/inspect.py", line 2424, in _signature_from_function
annotations = get_annotations(func, globals=globals, locals=locals, eval_str=eval_str)
File "/path/to/lib/python3.12/inspect.py", line 292, in get_annotations
value if not isinstance(value, str) else eval(value, globals, locals)
NameError: name 'AsyncGenerator' is not defined

Root Cause:

In Python 3.12, AsyncGenerator must be explicitly imported from typing or collections.abc. It seems FastAPI 0.124.0 (or one of its dependencies) uses AsyncGenerator in a type annotation as a string that gets evaluated at runtime via inspect.signature(call, eval_str=True), but the name is not available in the evaluation namespace.

Workaround:
Downgrading to FastAPI 0.115.6 resolves the issue:

pip install 'fastapi<0.124.0'

Steps to Reproduce:

1. Create a fresh virtual environment with Python 3.12
2. Install FastAPI 0.124.0: pip install fastapi==0.124.0
3. Install any application that uses FastAPI routes with dependencies that have AsyncGenerator type hints (e.g., Langflow 1.6.8)
4. Try to run the application

Expected Behavior:
Application should start without errors, as it did with FastAPI 0.115.x and earlier versions.

Actual Behavior:
Application crashes immediately on import with NameError: name 'AsyncGenerator' is not defined.

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.124.0 (latest)

Pydantic Version

2.11.10

Python Version

3.12

Additional Context

No response