InvalidRequestError: VARCHAR requires a length on dialect mysql

I am trying to create a remote database using mysql on an Ubuntu machine running 12.04.

It has a root user with remote login enabled and no password.I have started the server.

output of

sudo netstat -tap | grep mysql 

shows

tcp 0 0 localhost:mysql *:* LISTEN 13246/mysqld 

I have created a database called nwtopology using (as mentioned root doesn't have a password yet.)

 create database nwtopology grant all privileges on *.* to [email protected] FLUSH PRIVILEGES; 

From the client machine that also runs Ubuntu 12.04 I use a python script to connect to the remote mysql database using sqlalchemy.

from pox.core import core import pox.openflow.libopenflow_01 as of import re import datetime import time from sqlalchemy import create_engine, ForeignKey from sqlalchemy import Column, Date, Integer, String from sqlalchemy.ext.declarative import declarative_base from sqlalchemy.orm import relationship, backref from sqlalchemy import create_engine from sqlalchemy.orm import sessionmaker from sqlalchemy.sql.expression import exists log = core.getLogger() engine = create_engine('mysql://[email protected]/nwtopology', echo=False) Base = declarative_base() Session = sessionmaker(bind=engine) session = Session() class SourcetoPort(Base): """""" __tablename__ = 'source_to_port' id = Column(Integer, primary_key=True) port_no = Column(Integer) src_address = Column(String,index=True) #----------------------------------------- def __init__(self, src_address,port_no): """""" self.src_address = src_address self.port_no = port_no #create tables Base.metadata.create_all(engine) 

The last line

Base.metadata.create_all(engine) 

returns the error

 File "/usr/lib/python2.7/dist-packages/sqlalchemy/sql/compiler.py", line 1699, in visit_string return self.visit_VARCHAR(type_) File "/usr/lib/python2.7/dist-packages/sqlalchemy/dialects/mysql/base.py", line 1654, in visit_VARCHAR self.dialect.name) InvalidRequestError: VARCHAR requires a length on dialect mysql 

What does this mean? how can I set VARCHAR length on mysql? I am very new to sqlalchemy and mysql.