# -*- coding: utf-8 -*-
"""
    flaskext_compat
    ~~~~~~~~~~~~~~~

    Implements the ``flask.ext`` virtual package for versions of Flask
    older than 0.7.  This module is a noop if Flask 0.8 was detected.

    Usage::

        import flaskext_compat
        from flask.ext import foo

    :copyright: (c) 2011 by Armin Ronacher.
    :license: BSD, see LICENSE for more details.
"""
import sys
import imp


ext_module = imp.new_module('flask.ext')
ext_module.__path__ = []
ext_module.__package__ = ext_module.__name__


class _ExtensionImporter(object):
    """This importer redirects imports from the flask.ext module to other
    locations.
    """
    _module_choices = ['flask_%s', 'flaskext.%s']
    prefix = ext_module.__name__ + '.'
    prefix_cutoff = prefix.count('.')

    def find_module(self, fullname, path=None):
        if fullname.startswith(self.prefix):
            return self

    def load_module(self, fullname):
        if fullname in sys.modules:
            return sys.modules[fullname]
        modname = fullname.split('.', self.prefix_cutoff)[self.prefix_cutoff]
        for path in self._module_choices:
            realname = path % modname
            try:
                __import__(realname)
            except ImportError:
                exc_type, exc_value, tb = sys.exc_info()
                if self.is_important_traceback(realname, tb):
                    raise exc_type, exc_value, tb
                continue
            module = sys.modules[fullname] = sys.modules[realname]
            if '.' not in modname:
                setattr(ext_module, modname, module)
            return module
        raise ImportError('No module named %s' % fullname)

    def is_important_traceback(self, important_module, tb):
        """Walks a traceback's frames and checks if any of the frames
        originated in the given important module.  If that is the case
        then we were able to import the module itself but apparently
        something went wrong when the module was imported.  (Eg: import
        of an import failed).
        """
        while tb is not None:
            if tb.tb_frame.f_globals.get('__name__') == important_module:
                return True
            tb = tb.tb_next
        return False


def activate():
    """Activates the compatibility system."""
    import flask
    if hasattr(flask, 'ext'):
        return
    sys.modules['flask.ext'] = flask.ext = ext_module
    sys.meta_path.append(_ExtensionImporter())