Python Using mysql connection from super class

Question

Below is just example code that provides the error I am hoping to get help on fixing, or getting an understanding of a better way to write this. I have a mysql "super" class called mysql_connection. In this class, the connection to the database is made. I also have a few methods within it. One that simply runs "select version()" to show that the connection/query works. I then have a "chktable" method which in this example instantiates a new subclass called "table" which inherits the super class. After instantiating the class, I then call a method within the subclass which attempts to use the the query method in the superclass to run "show tables like 'tbl name'". This is where I get an error.

import mysql.connector
from mysql.connector import errorcode
from mysql.connector.cursor import MySQLCursor

class mysql_connection(object):
    def __init__(self, **kwargs):
        self.connection_options = {}
        self.connection_options['user'] = 'root'
        self.connection_options['password'] = ''
        self.connection_options['host'] = '192.168.33.10'
        self.connection_options['port'] = '3306'
        self.connection_options['database'] = "test"
        self.connection_options['raise_on_warnings'] = True
        self.connect()

    def connect(self):
        try:
            self.cnx = mysql.connector.connect(**self.connection_options)
        except mysql.connector.Error as err:
            if err.errno == errorcode.ER_ACCESS_DENIED_ERROR:
                print "Something is wrong with your user name or password"
            elif err.errno == errorcode.ER_BAD_DB_ERROR:
                print "Database does not exists" 
            else:
                print err

    def query(self, statement, data=''):
        cursor = MySQLCursor(self.cnx)
        cursor.execute(statement)
        result = cursor.fetchall()
        cursor.close
        return result

    def get_version(self):
        print self.query("select version()")

    def chktable(self, tb_name):
        tab = table(name=tb_name)
        tab.check_table()

class table(mysql_connection):
    def __init__(self, **kwargs):
        self.name = kwargs['name']

    def check_table(self):
        return super(table, self).query("show tables like '{}".format(self.name))

conn = mysql_connection()
conn.get_version()
conn.chktable("test")

The error that I get is:

$ python example.py
[(u'5.1.73',)]
Traceback (most recent call last):
  File "example.py", line 50, in <module>
    conn.chktable("test")
  File "example.py", line 39, in chktable
    tab.check_table()
  File "example.py", line 46, in check_table
    return super(table, self).query("show tables like '{}".format(self.name))
  File "example.py", line 28, in query
    cursor = MySQLCursor(self.cnx)
    AttributeError: 'table' object has no attribute 'cnx'

I don't fully understand calling back to the super class and how it works with subclasses, so that is likely my issue. I'd also like to know if there is maybe a better way to accomplish this. I was thinking I could get rid of the subclass altogether, but I like the subclass I made so I feel there should be a way around it. A secondary thing I could try would be to put the subclass inside the master class, but I don't think that is correct.

Why does your table inherit from the connection?!

jonrsharpe
– jonrsharpe

2014-12-06 09:04:14 +00:00
Commented Dec 6, 2014 at 9:04 — jonrsharpe
– jonrsharpe, Commented Dec 6, 2014 at 9:04

Daniel Roseman · Accepted Answer · 2014-12-06 09:55:56Z

2

As Jon points out, this is not an appropriate use of inheritance. That is for "is-a" relationships: ie Dog inherits from Animal, because a dog is an animal. But a table is not a connection: a table might use a connection, but that simply means you should assign an instance of the connection to an instance variable in table.

Also, in an inheritance relationship there is usually no good reason for a superclass to know about its subclasses, as you have in the chktable method.

(The actual bug you're seeing is because you haven't called the superclass method in table's __init__, but it's better to fix your structure.)

answered Dec 6, 2014 at 9:55

Daniel Roseman

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2 Comments

david Over a year ago

Thank you. You are correct that it was not an appropriate use of inheritance. Though this was only an example, and the mysql_connnection class is named differently in the real code, my class is a "has a" composition instead of a "is a." With further thought on my code, I have realized that I will not be using the "has a" class anywhere else, so I will be just adding it to the main class. I may go about making an actual mysql connection composition class to decouple it from my main mysql class I am working on.

nicholsonjf Over a year ago

Great explanation of when to use inheritance and how to handle this situation without it using and instance variable

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