id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
26463	Allowing Callbacks/Handlers to be called on Cache entry Expiration	Dylan Herman	Dylan Herman	"I read a suggestion about this here: [http://stackoverflow.com/questions/20866460/django-cache-backend-how-to-implement-callback-when-cache-timeout] .
 

I have written up an implementation of it with some tests for FileBasedCache (filebased.py) and want to add it to locmem.py too (not done  yet).

Here are code snippets from filebased.py that have been tested:
   The edits  are in set and _is_expired
{{{
 def set(self, key, value, timeout=DEFAULT_TIMEOUT, version=None,handler=None):
        self._createdir()  # Cache dir can be deleted at any time.
        fname = self._key_to_file(key, version)
        self._cull()  # make some room if necessary
        fd, tmp_path = tempfile.mkstemp(dir=self._dir)
        renamed = False
        try:
            with io.open(fd, 'wb') as f:
                expiry = self.get_backend_timeout(timeout)
                f.write(pickle.dumps(expiry, pickle.HIGHEST_PROTOCOL))
                #check if handler exists
                if handler:
                  #write handler to pickle
                  f.write(pickle.dumps(handler,pickle.HIGHEST_PROTOCOL))
                f.write(zlib.compress(pickle.dumps(value, pickle.HIGHEST_PROTOCOL)))
            file_move_safe(tmp_path, fname, allow_overwrite=True)
            renamed = True
        finally:
            if not renamed:
                os.remove(tmp_path)


 def _is_expired(self, f):
        """"""
        Takes an open cache file and determines if it has expired,
        deletes the file if it is has passed its expiry time.
        """"""
        exp = pickle.load(f)
        if exp is not None and exp < time.time():
            #if there is a handler, call it
            try:
              handler = pickle.load(f)
              handler(exp)
            except pickle.UnpicklingError:
             #handler not added(no handler specified), so pickling error occurs
              pass
            f.close()  # On Windows a file has to be closed before deleting
            self._delete(f.name)
            return True
        return False
}}}

The handler has the following format
     
{{{
        class Handler(object):
              def __call__(self,exp):
                    ...
}}}


"	New feature	closed	Core (Cache system)	dev	Normal	wontfix			Unreviewed	1	0	0	0	0	0