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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 to_datetime_field(formatter):
 """ Returns a callable instance that will convert a string to a DateTime. :param formatter: String that represents data format for parsing. :return: instance of the DateTimeConverter. """

    class DateTimeConverter(object):

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

        def __call__(self, value):
            if isinstance(value, string_types):
                value = parser.parse(value)

            return value

    return DateTimeConverter(formatter)