'Initialize a SwiftPackReader
:param scon: a `SwiftConnector` instance
:param filename: the pack filename'

def __init__(self, scon, filename):

    self.scon = scon
    self._filename = filename
    self._header_size = 12
    headers = self.scon.get_object_stat(self._filename)
    self.pack_length = int(headers['content-length'])
    pack_reader = SwiftPackReader(self.scon, self._filename, self.pack_length)
    (version, self._num_objects) = read_pack_header(pack_reader.read)
    self._offset_cache = LRUSizeCache(((1024 * 1024) * self.scon.cache_length), compute_size=_compute_object_size)
    self.pack = None
