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Python attribute error when using sqlite cursor from class

I am trying to fix an issue I am having but can't seem to find the issue. I need to create a Python class for handling database interactions. So far I have done something like this, but I am getting an AttributeError which I could not resolve.
import sqlite3
class Database:
def __init__(self):
self.file = 'database.db'
self.conn = sqlite3.connect(self.file)
self.cur = self.conn.cursor()
def query(self, query, *args):
try:
self.cur.execute(query, args)
self.conn.commit()
self.conn.close()
except sqlite3.Error as e:
print(e)
I then run the following command:
sql = 'INSERT INTO table (name) VALUES (?);'
Database.query(sql, "ABC")
But get this error message:
AttributeError: 'str' object has no attribute 'cur'
Please help me figure this out :D Thanks

You have to initiate your object before using it:
db = Database()
db.query(sql, "ABC")

How to initialize a database connection only once and reuse it in run-time in python?

I am currently working on a huge project, which constantly executes queries. My problem is, that my old code always created a new database connection and cursor, which decreased the speed immensivly. So I thought it's time to make a new database class, which looks like this at the moment:
class Database(object):
_instance = None
def __new__(cls):
if cls._instance is None:
cls._instance = object.__new__(cls)
try:
connection = Database._instance.connection = mysql.connector.connect(host="127.0.0.1", user="root", password="", database="db_test")
cursor = Database._instance.cursor = connection.cursor()
except Exception as error:
print("Error: Connection not established {}".format(error))
else:
print("Connection established")
return cls._instance
def __init__(self):
self.connection = self._instance.connection
self.cursor = self._instance.cursor
# Do database stuff here
The queries will use the class like so:
def foo():
with Database() as cursor:
cursor.execute("STATEMENT")
I am not absolutly sure, if this creates the connection only once regardless of how often the class is created. Maybe someone knows how to initialize a connection only once and how to make use of it in the class afterwards or maybe knows if my solution is correct. I am thankful for any help!

Explanation
The keyword here is clearly class variables. Taking a look in the official documentation, we can see that class variables, other than instance variables, are shared by all class instances regardless of how many class instances exists.
Generally speaking, instance variables are for data unique to each instance and class variables are for attributes and methods shared by all instances of the class:
So let us asume you have multiple instances of the class. The class itself is defined like below.
class Dog:
kind = "canine" # class variable shared by all instances
def __init__(self, name):
self.name = name # instance variable unique to each instance
In order to better understand the differences between class variables and instance variables, I would like to include a small example here:
>>> d = Dog("Fido")
>>> e = Dog("Buddy")
>>> d.kind # shared by all dogs
"canine"
>>> e.kind # shared by all dogs
"canine"
>>> d.name # unique to d
"Fido"
>>> e.name # unique to e
"Buddy"
Solution
Now that we know that class variables are shared by all instances of the class, we can simply define the connection and cursor like shown below.
class Database(object):
connection = None
cursor = None
def __init__(self):
if Database.connection is None:
try:
Database.connection = mysql.connector.connect(host="127.0.0.1", user="root", password="", database="db_test")
Database.cursor = Database.connection.cursor()
except Exception as error:
print("Error: Connection not established {}".format(error))
else:
print("Connection established")
self.connection = Database.connection
self.cursor = Database.cursor
As a result, the connection to the database is created once at the beginning and can then be used by every further instance.

Kind of like this. It's a cheap way of using a global.
class Database(object):
connection = None
def __init__(self):
if not Database.connection:
Database.connection = mysql.connector.connect(host="127.0.0.1", user="root", password="", database="db_test")
def query(self,sql):
cursor = Database.connection.cursor()
cursor.execute(sql)
# Do database stuff here

This too does work and you are guaranteed to always have one instance of the database
def singleton(class_):
instances = {}
def get_instance(*args, **kwargs):
if class_ not in instances:
instances[class_] = class_(*args, **kwargs)
return instances[class_]
return get_instance
#singleton
class SingletonDatabase:
def __init__(self) -> None:
print('Initializing singleton database connection... ', random.randint(1, 100))

The Reason you have to do all this is if you just create
a connection once and leave it at that you then
will end up trying to use a connection which is dropped
so you create a connection and attach it to your app
then whenever you get a new request check if the connection
still exists, with before request hook if not then recreate the
connection and proceeed.
on create_app
def create_app(self):
if not app.config.get('connection_created'):
app.database_connection = Database()
app.config['connection_created'] = True
on run app
#app.before_request
def check_database_connection(self):
if not app.config.get('connection_created') or not app.database_connection:
app.database_connection = Database()
app.config['connection_created'] = True
this will insure that your application always runs with an active connection
and that it gets created only once per app
if connection is dropped on any subsequent call then it gets recreated again...

How to extend OracleCursor class from cx_Oracle

Using Python 2.7.12 and package cx_Oracle I'm trying to create an extended class of the what the package call OracleCursor. I simply want to inherit the methods from the superclass and extend with some custom methods.
First I get the OracleCursor by
import cx_Oracle
conn = cx_Oracle.connect(username, password, dsn)
cursor = conn.cursor()
and I then have the following
>>> type(cursor)Out[6]:
OracleCursor
>>> isinstance(cursor, cx_Oracle.Cursor)
True
One would think that it is achieved by
class ExtendedCursor(cx_Oracle.Cursor):
def hello_world(self):
print('Hello world')
extended = ExtendedCursor(cursor)
but I get TypeError: argument 1 must be cx_Oracle.Connection, not OracleCursor. To me that error doesn't make sense. Also, I can't use OracleCursor as my superclass since it isn't recognized as a class.

The cursor is returned from the Connection object. You need to create a custom connection that returns your ExtendedCursor.
import cx_Oracle as cxo
class MyCursor(cxo.Cursor):
def helloWorld(self):
print "helloWorld"
class MyConnection(cxo.Connection):
def cursor(self):
return MyCursor(self)
if __name__ == '__main__':
conStr = '<user>/<password>#127.0.0.1:1521/xe'
db = MyConnection(conStr)
c = db.cursor()
print c
c.execute('select 1+1 from dual')
print(c.fetchall())
c.helloWorld()
returns:
<__main__.MyCursor on <__main__.MyConnection to ...#127.0.0.1:1521/xe>>
[(2,)]
helloWorld

Python Using mysql connection from super class

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.)

How can I inherit from psycopg2?

I'm trying inherit psycopg2 like this:
import psycopg2
class myp(psycopg):
pass
ii = myp
ii.connect(database = "myDataBase", user = "myUser", password="myPassword")
Then it throws an error:
class myp(psycopg2._psycopg):
TypeError: Error when calling the metaclass bases
module.__init__() takes at most 2 arguments (3 given)
Is it possible to inherit from psycopg2 library?
EDIT:
I want to use different databases, so I just have to change the class MyDatabase. something like a wrapper.
example:
import psycopg2
class MyDatabase(psycopg2):
def connect(self):
#do some stuff
return psycopg2.connect(database = "myDataBase", user = "myUser", password="myPassword")
for mysqldb
import MySQLdb
class MyDatabase(MySQLdb)
def connect(self):
#do some stuff
return psycopg2.connect(database = "myDataBase", user = "myUser", password="myPassword")
and derived class
class MyDataBaseApp(MyDatabase):
def add(self, myObjectClass):
db = MyDatabase()
cn = None
try:
cn = db.connect()
cur = cn.cursor()
cur.execute ("INSERT ...",(myObjectClass.parameter1, myObjectClass.parameter2))
cn.commit()
except MyDatabase.DatabaseError, e:
print e
if cn:
cn.rollback()
finally:
if cn:
cn.close()
but according to the documentation I have to do it another way, suggestions?

Disclaimer: I'm not familiar with psycopg
Update
Seems the documentation recommends to subclass psycopg2.extensions.connection. Then, connect() is a factory function that can still be used to create new connections, but you have to provide your class as a factory, again according to the docs
Full code may have to look more like (untested):
import psycopg2
class myp(psycopg2.extensions.connection):
pass
ii = connect(connection_factory=myp,
database = "myDataBase", user = "myUser", password="myPassword")
Update 2
With the updated approach, you're trying to build new classes with different/divergent interfaces. Often, composition is better than inheritance, see wikipedia and this question.