<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def to_time_field(formatter):
 """ Returns a callable instance that will convert a string to a Time. :param formatter: String that represents data format for parsing. :return: instance of the TimeConverter. """

    class TimeConverter(object):

        def __init__(self, formatter):
            self.formatter = formatter

        def __call__(self, value):
            if isinstance(value, string_types):
                value = datetime.strptime(value, self.formatter).time()

            return value

    return TimeConverter(formatter)