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.
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.)