Real-time YAML object access in Python

yaml_file.py

"""
liveyamlfile.py
Live Pythonic attribute access to properties in a yaml file.
by Peter Sobot (hi@psobot.com), August 8, 2012
"""

import os
import time
import yaml
import logging


class LiveYamlFile(object):
    """
    Magical class that allows for real-time access of variables in a yaml
    file via Pythonic attribute-like access.

    Any functions, objects or properties on the class (or its subclasses)
    will not be looked up in the yaml file. Every access may result in at
    worst, a YAML load and at best, an os.stat call at most every __timeout
    seconds.

    Simple example usage:
        > my_object = yaml_file("file.yml")
        > my_object.some_changing_property
        5
        > # (now here, file.yml has changed outside of python)
        > my_object.some_changing_property
        6
        > my_object.blah     # blah is not defined in the yaml file
        AttributeError: object my_object has no attribute 'blah'
    """

    __last_updated = 0
    __timeout = 5  # seconds
    __exclude = []

    def __init__(self, filename):
        self.__file = filename

        #   Any subclass's properties will be ignored here
        supers = LiveYamlFile.__dict__
        subs = self.__class__.__dict__
        self.__exclude = [k for k, _ in
                          dict(supers.items() + subs.items()).iteritems()
                          if not k.startswith("_")]

    def update(self):
        """
        Update the object's attributes from the YAML file.
        """
        target_file = open(self.__file)
        for attr in dir(self):
            if not attr.startswith("_") and attr not in self.__exclude:
                delattr(self, attr)
        for key, val in yaml.load(target_file).iteritems():
            if not key.startswith("_") and key not in self.__exclude:
                setattr(self, key, val)
        target_file.close()
        if hasattr(self, 'log_config_file_changes')\
                and self.log_config_file_changes:
            logging.getLogger(__name__).info("Config file has updated.")

    def __getattribute__(self, name):
        """
        When trying to access an attribute, check if the underlying file
        has changed first. Best case: reads from Python cache. Worst case:
        performs an os.stat and a YAML load every __timeout seconds.
        """
        if name.startswith("_") or name in self.__exclude:
            return object.__getattribute__(self, name)
        else:
            last_updated = self.__last_updated

            #   Only check once every __timeout seconds
            if (time.time() - last_updated) > self.__timeout:
                fmod_time = os.stat(self.__file)[9]
                if last_updated < fmod_time:
                    self.__last_updated = fmod_time
                    self.update()
            return object.__getattribute__(self, name)