from jinja2 import loaders


def jinja_loader_from_django_loader(django_loader):
    """Attempts to make a conversion from the given Django loader to an
    similarly-behaving Jinja loader.

    :param django_loader: Django loader module string.
    :return: The similarly-behaving Jinja loader, or None if a similar loader
        could not be found.
    """
    for substr, func in _JINJA_LOADER_BY_DJANGO_SUBSTR.iteritems():
        if substr in django_loader:
            return func()
    return None


def _make_jinja_app_loader():
    """Makes an 'app loader' for Jinja which acts like
    :mod:`django.template.loaders.app_directories`.
    """
    from django.template.loaders.app_directories import app_template_dirs
    return loaders.FileSystemLoader(app_template_dirs)


def _make_jinja_filesystem_loader():
    """Makes a 'filesystem loader' for Jinja which acts like
    :mod:`django.template.loaders.filesystem`.
    """
    from django.conf import settings
    return loaders.FileSystemLoader(settings.TEMPLATE_DIRS)


# Determine loaders from Django's conf.
_JINJA_LOADER_BY_DJANGO_SUBSTR = { # {substr: callable, ...}
    'app_directories': _make_jinja_app_loader,
    'filesystem': _make_jinja_filesystem_loader,
}