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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 field_for(self, field_id):
 """Fetches the field for the given Field ID. :param field_id: ID for Field to fetch. :return: :class:`ContentTypeField <ContentTypeField>` object. :rtype: contentful.ContentTypeField """


        for field in self.fields:
            if field.id == field_id:
                return field
        return None