OpenBB-finance/OpenBB · error · TypeError

Error: The {name} instance in '{app_path}' appears not to be

Error message

Error: The {name} instance in '{app_path}' appears not to be a callable factory function

What it means

TypeError raised when the --factory flag is set, the named attribute was found, but calling it raised TypeError — which the importer interprets as 'this attribute is not a callable factory function' (or its signature is not zero-argument callable). The same try/except otherwise silently tolerates calling a non-factory app instance.

Source

Thrown at openbb_platform/extensions/mcp_server/openbb_mcp_server/utils/app_import.py:92

        raise AttributeError(
            f"Error: The app file '{app_path}' does not contain an '{name}' instance"
        )

    app_or_factory = getattr(module, name)

    # Here we use the same approach as uvicorn to handle factory functions.
    # This prevents us from relying on explicit type annotations.
    # See: https://github.com/encode/uvicorn/blob/master/uvicorn/config.py
    try:
        app = app_or_factory()
        if not factory:
            print(  # noqa: T201
                "\n\n[WARNING]   "
                "App factory detected. Using it, but please consider setting the --factory flag explicitly.\n"
            )
    except TypeError:
        if factory:
            raise TypeError(  # pylint: disable=raise-missing-from
                f"Error: The {name} instance in '{app_path}' appears not to be a callable factory function"
            )
        app = app_or_factory

    if not isinstance(app, FastAPI):
        raise TypeError(
            f"Error: The {name} instance in '{app_path}' is not an instance of FastAPI"
        )

    return app


cl_doc = """OpenBB MCP Server

Usage:
    >>> python -m openbb_mcp_server [OPTIONS]

    >>> openbb-mcp --app ./some_app.py --host 0.0.0.0 --port 8005

View on GitHub (pinned to 3e071fcc2c)

Solutions

  1. If the target is already a FastAPI instance, drop --factory
  2. If it is a real factory, make it zero-argument callable (read its config from environment/settings inside the function)
  3. Ensure --name points at the function itself, not at something it returns or wraps

Example fix

# before
def create_app(settings: Settings) -> FastAPI: ...
openbb-mcp --app main.py:create_app --factory

# after
def create_app() -> FastAPI:
    settings = Settings()
    ...
openbb-mcp --app main.py:create_app --factory
Defensive patterns

Strategy: type-guard

Validate before calling

import inspect

target = getattr(module, name)
if factory:
    if not callable(target):
        raise ValueError(f"--factory set but {name!r} is not callable")
    try:
        inspect.signature(target).bind()
    except TypeError:
        raise ValueError(f"factory {name!r} must be callable with zero arguments")

Type guard

def is_zero_arg_factory(module, name: str) -> bool:
    import inspect
    target = getattr(module, name, None)
    if not callable(target):
        return False
    try:
        inspect.signature(target).bind()
        return True
    except TypeError:
        return False

Try / catch

try:
    app = import_app("./main.py", "create_app", True)
except TypeError as e:
    if "not to be a callable factory" in str(e):
        app = import_app("./main.py", "create_app", False)  # it is an instance
    else:
        raise

Prevention

When it happens

Trigger: --app main.py:create_app --factory where create_app actually requires arguments (create_app(settings)); or the attribute is a plain FastAPI instance (not callable) while --factory was passed; or a factory with a typo'd required parameter.

Common situations: Copy-pasting a --factory command line against code whose factory needs configuration objects, flagging --factory 'just in case' when the target is already an instance, refactoring a factory to take arguments after the launch script was written.

Related errors

AI-assisted analysis of OpenBB-finance/OpenBB@3e071fcc2c (2026-08-14). Data as JSON: /api/errors/0ff24bcf19a62847. Report an issue: GitHub.