"""
Wrapper for loading templates from "templates" directories in INSTALLED_APPS
packages.
"""

import os
import sys

from django.conf import settings
from django.core.exceptions import ImproperlyConfigured
from django.template.base import TemplateDoesNotExist
from django.template.loader import BaseLoader
from django.utils._os import safe_join
from django.utils.importlib import import_module
from django.utils import six

# At compile time, cache the directories to search.
if six.PY2:
    fs_encoding = sys.getfilesystemencoding() or sys.getdefaultencoding()
app_template_dirs = []
for app in settings.INSTALLED_APPS:
    try:
        mod = import_module(app)
    except ImportError as e:
        raise ImproperlyConfigured('ImportError %s: %s' % (app, e.args[0]))
    template_dir = os.path.join(os.path.dirname(mod.__file__), 'templates')
    if os.path.isdir(template_dir):
        if six.PY2:
            template_dir = template_dir.decode(fs_encoding)
        app_template_dirs.append(template_dir)

# It won't change, so convert it to a tuple to save memory.
app_template_dirs = tuple(app_template_dirs)

class Loader(BaseLoader):
    is_usable = True

    def get_template_sources(self, template_name, template_dirs=None):
        """
        Returns the absolute paths to "template_name", when appended to each
        directory in "template_dirs". Any paths that don't lie inside one of the
        template dirs are excluded from the result set, for security reasons.
        """
        if not template_dirs:
            template_dirs = app_template_dirs
        for template_dir in template_dirs:
            try:
                yield safe_join(template_dir, template_name)
            except UnicodeDecodeError:
                # The template dir name was a bytestring that wasn't valid UTF-8.
                raise
            except ValueError:
                # The joined path was located outside of template_dir.
                pass

    def load_template_source(self, template_name, template_dirs=None):
        for filepath in self.get_template_sources(template_name, template_dirs):
            try:
                with open(filepath, 'rb') as fp:
                    return (fp.read().decode(settings.FILE_CHARSET), filepath)
            except IOError:
                pass
        raise TemplateDoesNotExist(template_name)
