import json
import time
import logging
from collections import UserDict

class AutoReloadConfig(UserDict):
    """
    A dictionary that automatically reloads itself from a file.
    On each call to __getitem__(), check if config is older than reload_period and if yes then reload from file.

    Example:
        # load from config.json every 60s
        config = AutoReloadConfig("config.json", 60)

        # gets value from config (and reloads beforehand if last get was before 60s)
        test = config["test"]
    """
    def __init__(self, config_filename, reload_period = 60):
        self._logger = logging.getLogger("config")

        self._reload_period = reload_period
        self._filename = config_filename

        self._age = 0

    def __getitem__(self, key):
        if self._age + self._reload_period < time.time():
            self._reload()
        return super().__getitem__(key)

    def _reload(self):
        try:
            with open(self._filename) as fp:
                config = json.load(fp)

                self._age = time.time()

                # set data used by UserDict
                self.data = config
            self._logger.debug("config reloaded")
        except:
            self._logger.exception("failed to reload configuration from file \"" + self._filename + "\".")