Reads the cache file as pickle file.

def read(self):
        """Reads the cache file as pickle file."""

        def warn(msg, elapsed_time, current_time):
            desc = self._cache_id_desc()
            self._warnings(
                "{0} {1}: {2}s < {3}s", msg, desc, elapsed_time, current_time)

        file_time = get_time()
        out = self._out
        if out is None:
            if self.verbose:
                self._warnings("reading {0} from disk", self._cache_id_desc())
            with open(self._cache_file, 'rb') as f_in:
                out = None
                while True:
                    t_out = f_in.read(CHUNK_SIZE)
                    if not len(t_out):
                        break
                    if out is not None:
                        out += t_out
                    else:
                        out = t_out
                self._out = out
        (cache_id_obj, elapsed_time, res) = self._read(out)
        self.ensure_cache_id(cache_id_obj)
        real_time = get_time() - file_time
        if elapsed_time is not None and real_time > elapsed_time:
            warn("reading cache from disk takes longer than computing!",
                 elapsed_time, real_time)
        elif self._start_time is not None and elapsed_time is not None:
            current_time = get_time() - self._start_time
            if elapsed_time < current_time:
                warn("reading cache takes longer than computing!",
                     elapsed_time, current_time)
        self._last_access = get_time()
        return res