mypy: Blueprint type mismatch with `flask.sansio.blueprints.Blueprint`
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Assessment
- Difficulty
- 5/5
- Estimated time
- Over a week
- Newbie friendliness
- 35/100
- Issue type
- Bug
- Clarity
- Needs clarification
- Activity status
- Stale
- Domain
- backend, developer-experience
Research direction
Start with the Quart Blueprint and Quart entry points that expose app.blueprints, then compare their annotations with Flask's sansio Blueprint definitions. Use the provided minimal example with mypy --strict to evaluate whether the Flask typing or a Quart-side cast best resolves the mismatch, and consider the impact on the Quart/Flask ecosystem before defining done.
Written by the indexing model from the issue text.
Description
When using Quart's Blueprint, mypy doesn't recognize its inheritance from flask.sansio.blueprints.Blueprint. This causes type errors when accessing blueprints through app.blueprints, which returns the Flask Blueprint type.
Environment
- Python version: 3.12.7
- Quart version: 0.20.0
Minimal Reproducible Example
from quart import Quart
from quart.blueprints import Blueprint
app = Quart(__name__)
bp = Blueprint("test", __name__)
app.register_blueprint(bp)
# This works at runtime but fails type check
def process_blueprint(blueprint: Blueprint) -> None:
print(f"Processing blueprint: {blueprint.name}")
# Type error here - blueprints.values() returns flask.sansio.blueprints.Blueprint
for blueprint in app.blueprints.values():
process_blueprint(blueprint) # Error: Expected quart.blueprints.Blueprint, got flask.sansio.blueprints.Blueprint
# Show inferred types
reveal_type(bp) # Shows quart.blueprints.Blueprint
reveal_type(app.blueprints) # Shows Dict[str, flask.sansio.blueprints.Blueprint]
Mypy output:
mypy quart_blueprint_mre.py --show-error-codes --strict
quart_blueprint_mre.py:19: error: Argument 1 to "process_blueprint" has incompatible type "flask.sansio.blueprints.Blueprint"; expected "quart.blueprints.Blueprint" [arg-type]
quart_blueprint_mre.py:22: note: Revealed type is "quart.blueprints.Blueprint"
quart_blueprint_mre.py:23: note: Revealed type is "builtins.dict[builtins.str, flask.sansio.blueprints.Blueprint]"
Found 1 error in 1 file (checked 1 source file)
In my real codebase, I have something like this:
from typing import TypeAlias
from quart import Quart
from quart.blueprints import Blueprint
Scaffold: TypeAlias = Blueprint | Quart
def _inject_routes(app: Scaffold) -> None:
for endpoint, func in app.view_functions.items():
if not injected(func):
wrapped = _make_wrapper(func)
app.view_functions[endpoint] = wrapped
It can be fixed, of course, with suppression or with adding flask.sansio.blueprints.Blueprint to the type alias, but I guess it's not the best solution.
As I found out, that's because of the more complex hierarchy of the App/Blueprint classes for Quart. So I'm wondering what would be the best way to handle this inheritance in the type system:
- Make Flask's sansio layer more generic to support framework extension:
TBlueprint = TypeVar('TBlueprint', bound='Blueprint', covariant=True)
blueprints: Dict[str, TBlueprint]
- Or add type casting on Quart's side:
@property
def blueprints(self) -> Dict[str, Blueprint]:
return cast(Dict[str, Blueprint], super().blueprints)
I'd be happy to submit a PR with either approach once you suggest me which is more appropriate for the Quart/Flask ecosystem.
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