core.py

class Singleton(object):
    """
    A non-thread-safe helper class to ease implementing singletons.
    This should be used as a decorator -- not a metaclass -- to the
    class that should be a singleton.

    The decorated class can define one `__init__` function that
    takes only the `self` argument. Other than that, there are
    no restrictions that apply to the decorated class.

    To get the singleton instance, use the `Instance` method. Trying
    to use `__call__` will result in a `TypeError` being raised.

    Limitations: The decorated class cannot be inherited from.

    """

    def __init__(self, decorated):
        self._decorated = decorated

    def Instance(self):
        """
        Returns the singleton instance. Upon its first call, it creates a
        new instance of the decorated class and calls its `__init__` method.
        On all subsequent calls, the already created instance is returned.

        """
        try:
            return self._instance
        except AttributeError:
            self._instance = self._decorated()
            return self._instance

    def __call__(self):
        raise TypeError('Singletons must be accessed through `Instance()`.')

    def __instancecheck__(self, inst):
        return isinstance(inst, self._decorated)

class _const:
    class ConstError(TypeError): pass
    def __setattr__(self,name,value):
        if self.__dict__.has_key(name):
            raise self.ConstError, "Can't rebind const(%s)"%name
        self.__dict__[name]=value

import threading
def run_in_thread(fn):
    t = threading.Thread(target=fn)
    t.daemon = True
    t.start()
    return t

import sys
sys.modules["const"]=_const()

def get_file_suffix(url):
    return url.split(".")[-1].split("?")[0]