code: def truncate_to_field_length(self, field, value):
          
          max_len = getattr(self.__class__, field).prop.columns[0].type.length
          if value and len(value) > max_len:
              return value[:max_len]
          else:
              return value
docs: Truncate the value of a string field to the field's max length.
  
          Use this in a validator to check/truncate values before inserting them into the database.
          Copy the below example code after ``@validates`` to your model class and replace ``field1`` and ``field2`` with
          your field name(s).
  
          :Example:
  
              from sqlalchemy.orm import validates
              # ... omitting other imports ...
  
              class MyModel(base.Base):
  
                  field1 = Column(String(128))
                  field2 = Column(String(64))
  
                  @validates('field1', 'field2')
                  def truncate(self, field, value):
                      return self.truncate_to_field_length(field, value)
  
          Args:
              field (str): field name to validate
              value (str/unicode): value to validate
  
          Returns:
              str/unicode: value truncated to field max length