see samuelcolvin/harrier this should one day be a package of its own

useharrier.py

import json
from time import time
import urllib.request


class UseHarrierError(Exception):
    pass


class UseHarrierKeyError(KeyError):
    pass


class HarrierStaticFilter:
    _update_t = 0
    _file_map = {}

    def __init__(self, file_map_ref, max_age=10):
        self._file_map_ref = file_map_ref
        self._max_age = max_age
        self._check_file_map()

    def __call__(self, file_key):
        self._check_file_map()
        f = self._file_map.get(file_key.strip(' /'))
        if f:
            return f
        raise UseHarrierKeyError('file "{}" not found, consult the file map "{}"'.format(file_key, self._file_map_ref))

    def _check_file_map(self):
        if (time() - self._update_t) < self._max_age:
            return
        with urllib.request.urlopen(self._file_map_ref) as r:
            if r.status != 200:
                raise UseHarrierError('bad response {} from "{}"'.format(r.status, self._file_map_ref))
            data = r.read()

        try:
            obj = json.loads(data.decode('utf8'))
        except json.JSONDecodeError as e:
            raise UseHarrierError('Error loading json: {}'.format(e))

        self._file_map = obj['files']
        self._update_t = time()