flask_failsafe.py

import functools
import sys
import traceback

import flask


PY2 = sys.version_info[0] == 2


def failsafe(func):
    """
    Wraps an app factory to provide a fallback in case of import errors.

    Takes a factory function to generate a Flask app. If there is an error
    creating the app, it will return a dummy app that just returns the Flask
    error page for the exception.

    This works with the Flask code reloader so that if the app fails during
    initialization it will still monitor those files for changes and reload
    the app.
    """

    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        extra_files = []

        try:
            return func(*args, **kwargs)
        except:
            exc_type, exc_val, exc_tb = sys.exc_info()
            traceback.print_exc()

        tb = exc_tb
        while tb:
            filename = tb.tb_frame.f_code.co_filename
            extra_files.append(filename)
            tb = tb.tb_next

        if isinstance(exc_val, SyntaxError):
            extra_files.append(exc_val.filename)

        app = _FailSafeFlask(extra_files)
        app.debug = True

        @app.route('/')
        @app.route('/<path:path>')
        def index(path='/'):
            reraise(exc_type, exc_val, exc_tb)

        return app

    return wrapper


if PY2:
    exec('def reraise(tp, value, tb=None):\n raise tp, value, tb')
else:
    def reraise(tp, value, tb=None):
        if value.__traceback__ is not tb:
            raise value.with_traceback(tb)
        raise value


class _FailSafeFlask(flask.Flask):
    """
    Binds the extra_args parameter of run() to include the extra files we want
    to monitor for changes.
    """

    def __init__(self, extra_files):
        flask.Flask.__init__(self, __name__)
        self.extra_files = extra_files

    def run(self, *args, **kwargs):
        extra_files = self.extra_files
        if 'extra_files' in kwargs:
            extra_files = extra_files + kwargs['extra_files']
        kwargs['extra_files'] = extra_files
        flask.Flask.run(self, *args, **kwargs)