'Establish instance variables with default values'

def __init__(self):

    self._defaults = (('xml', 'application/xml'), ('jpeg', 'image/jpeg'))
    self._empty = False
    self._overrides = (('/docProps/core.xml', 'app/vnd.type1'), ('/ppt/presentation.xml', 'app/vnd.type2'), ('/docProps/thumbnail.jpeg', 'image/jpeg'))
