from os.path import dirname, join, abspath, isdir

from django.db.models import get_app
from django.core.exceptions import ImproperlyConfigured
from django.template import TemplateDoesNotExist
from django.template.loaders.filesystem import load_template_source


def _get_template_vars(template_name):
    app_name, template_name = template_name.split(":", 1)
    try:
        template_dir = abspath(join(dirname(get_app(app_name).__file__), 'templates'))
    except ImproperlyConfigured:
        raise TemplateDoesNotExist()

    return template_name, template_dir

def load_template_from_app(template_name, template_dirs=None):
    """
    Template loader that only serves templates from specific app's template directory.

    Works for template_names in format app_label:some/template/name.html
    """
    if ":" not in template_name:
        raise TemplateDoesNotExist()

    template_name, template_dir = _get_template_vars(template_name)

    if not isdir(template_dir):
        raise TemplateDoesNotExist()

    return load_template_source(template_name, template_dirs=[template_dir])

load_template_from_app.is_usable = True
