pallets/flask · error · NoAppException

Could not locate a Flask application. Use the 'flask --app'…

Error message

Could not locate a Flask application. Use the 'flask --app' option, 'FLASK_APP' environment variable, or a 'wsgi.py' or 'app.py' file in the current directory.

What it means

ScriptInfo.load_app() tries, in order: create_app callback, the --app/FLASK_APP import path, then the default files wsgi.py and app.py in the current directory (with raise_if_not_found=False so a missing default is silent). If after all of these app is still None, Flask raises NoAppException telling you how to specify the app.

Solutions

1. Set FLASK_APP to your app location: export FLASK_APP='mypkg'.
2. Use the --app CLI flag: flask --app mypkg run.
3. Create an app.py or wsgi.py in the project root that imports and exposes the Flask app instance.

Example fix

# before: app lives in mypkg/__init__.py, no app.py in cwd
flask run   # -> Could not locate a Flask application

# after
export FLASK_APP='mypkg'
flask run

import os

def flask_app_locatable() -> bool:
    if os.environ.get('FLASK_APP'):
        return True
    return os.path.exists('app.py') or os.path.exists('wsgi.py')

null

from flask.cli import NoAppException
try:
    script_info.load_app()
except NoAppException:
    os.environ['FLASK_APP'] = 'mypkg'
    raise

When it happens

Trigger: Running any flask CLI command (flask run, flask routes, etc.) without --app/FLASK_APP set and without a wsgi.py or app.py file in the current directory, and no create_app registered on ScriptInfo. Each default-file locate_app returned None because raise_if_not_found=False suppressed the ImportError.

Common situations: New project where the app lives in mypkg/__init__.py but no app.py/wsgi.py exists in cwd. Renaming the entry file so the auto-detection no longer finds it. Forgetting to export FLASK_APP in a new shell session.

Related errors

• A valid Flask application was not obtained from
• Could not import .
• The environment variable
• The factory in module could not be called with the…
• Allowed methods must be a list of strings, for example…

AI-assisted analysis of pallets/flask@3596b1ab61 (2026-08-11). Data as JSON: /api/errors/141828d6ec4dda64. Report an issue: GitHub.

Appendix: source

Thrown at src/flask/cli.py:359

        if self.create_app is not None:
            app = self.create_app()
        else:
            if self.app_import_path:
                path, name = (
                    re.split(r":(?![\\/])", self.app_import_path, maxsplit=1) + [None]
                )[:2]
                import_name = prepare_import(path)
                app = locate_app(import_name, name)
            else:
                for path in ("wsgi.py", "app.py"):
                    import_name = prepare_import(path)
                    app = locate_app(import_name, None, raise_if_not_found=False)

                    if app is not None:
                        break

        if app is None:
            raise NoAppException(
                "Could not locate a Flask application. Use the"
                " 'flask --app' option, 'FLASK_APP' environment"
                " variable, or a 'wsgi.py' or 'app.py' file in the"
                " current directory."
            )

        if self.set_debug_flag:
            # Update the app's debug flag through the descriptor so that
            # other values repopulate as well.
            app.debug = get_debug_flag()

        self._loaded_app = app
        return app


pass_script_info = click.make_pass_decorator(ScriptInfo, ensure=True)

F = t.TypeVar("F", bound=t.Callable[..., t.Any])

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