pallets/flask · error · NoAppException

Detected factory ' ' in module ' ', but could not call it…

Error message

Detected factory '{attr_name}' in module '{module.__name__}', but could not call it without arguments. Use '{module.__name__}:{attr_name}(args)' to specify arguments.

What it means

Raised when Flask's CLI finds a factory function named create_app or make_app in the module but calling it with no arguments raises a TypeError attributable to a wrong call signature (not an internal error). The factory needs arguments the CLI cannot infer.

Solutions

1. Pass arguments via FLASK_APP syntax: export FLASK_APP='module:create_app("production")'.
2. Give the factory arguments defaults so it can be called with no args.
3. Provide a no-arg wrapper or environment-driven factory.

Example fix

// before
def create_app(config_name: str): ...
# FLASK_APP=module -> raises
// after
# export FLASK_APP='module:create_app("production")'
# or give a default:
def create_app(config_name: str = 'development'): ...

import inspect
def factory_callable_no_args(f):
    try:
        inspect.signature(f).bind()
        return True
    except TypeError:
        return False

import inspect
def factory_takes_no_args(f) -> bool:
    params = inspect.signature(f).parameters
    return all(p.default is not p.empty for p in params.values())

try:
    app = create_app()
except TypeError:
    # supply args via FLASK_APP='module:create_app("prod")'
    raise

When it happens

Trigger: def create_app(config_name): ... declared in the module, FLASK_APP=module with no args supplied; the CLI calls create_app() and the missing positional arg triggers TypeError detected by _called_with_wrong_args.

Common situations: Application factory that requires a config name or environment argument; refactored factory signature not yet reflected in FLASK_APP.

Related errors

• Detected multiple Flask applications in module
• Failed to find attribute
• Failed to find Flask application or factory in module
• Failed to parse as an attribute name or function call.
• Failed to parse arguments as literal values

AI-assisted analysis of pallets/flask@3596b1ab61 (2026-08-11). Data as JSON: /api/errors/0f141fde0a69a428. Report an issue: GitHub.

Appendix: source

Thrown at src/flask/cli.py:80

            f" '{module.__name__}'. Use '{module.__name__}:name'"
            " to specify the correct one."
        )

    # Search for app factory functions.
    for attr_name in ("create_app", "make_app"):
        app_factory = getattr(module, attr_name, None)

        if inspect.isfunction(app_factory):
            try:
                app = app_factory()

                if isinstance(app, Flask):
                    return app
            except TypeError as e:
                if not _called_with_wrong_args(app_factory):
                    raise

                raise NoAppException(
                    f"Detected factory '{attr_name}' in module '{module.__name__}',"
                    " but could not call it without arguments. Use"
                    f" '{module.__name__}:{attr_name}(args)'"
                    " to specify arguments."
                ) from e

    raise NoAppException(
        "Failed to find Flask application or factory in module"
        f" '{module.__name__}'. Use '{module.__name__}:name'"
        " to specify one."
    )


def _called_with_wrong_args(f: t.Callable[..., Flask]) -> bool:
    """Check whether calling a function raised a ``TypeError`` because
    the call failed or because something in the factory raised the
    error.

View on GitHub (pinned to 3596b1ab61)