code: def infix_filename(self, name, default, infix, ext=None):
          
          if name is None:
              p, oldext = os.path.splitext(default)
              if ext is None:
                  ext = oldext
              if ext.startswith(os.extsep):
                  ext = ext[1:]
              name = self.filename(p+infix, ext=ext)
          return name
docs: Unless *name* is provided, insert *infix* before the extension *ext* of *default*.