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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _convert(val, acceptable_types):
 """Ensure that val is one of the acceptable types, converting it if needed. :param val: The value we're parsing (either a string or one of the acceptable types). :param acceptable_types: A tuple of expected types for val. :returns: The parsed value. :raises :class:`pants.options.errors.ParseError`: if there was a problem parsing the val as an acceptable type. """

  if isinstance(val, acceptable_types):
    return val
  return parse_expression(val, acceptable_types, raise_type=ParseError)