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.
Related
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...
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
How I can pass the data from object oriented programming to mysql in python? Do I need to make connection in every class?
Update:
This is my object orinted
class AttentionDataPoint(DataPoint):
def __init__ (self, _dataValueBytes):
DataPoint._init_(self, _dataValueBytes)
self.attentionValue=self._dataValueBytes[0]
def __str__(self):
if(self.attentionValue):
return "Attention Level: " + str(self.attentionValue)
class MeditationDataPoint(DataPoint):
def __init__ (self, _dataValueBytes):
DataPoint._init_(self, _dataValueBytes)
self.meditationValue=self._dataValueBytes[0]
def __str__(self):
if(self.meditationValue):
return "Meditation Level: " + str(self.meditationValue)
And I try to get the data to mysql using this coding.
import time
import smtplib
import datetime
import MySQLdb
db = MySQLdb.connect("192.168.0.101", "fyp", "123456", "system")
cur = db.cursor()
while True:
Meditation_Level = meditationValue()
Attention_Level = attentionValue()
current_time = datetime.datetime.now()
sql = "INSERT INTO table (id, Meditation_Level, Attention_Level, current_time) VALUES ('test1', %s, %s, %s)"
data = (Meditation_Level, Attention_Level, current_time)
cur.execute(sql, data)
db.commit()
db.close()
Update:
DataPoint
class DataPoint:
def __init__(self, dataValueBytes):
self._dataValueBytes = dataValueBytes
I am thinking of something like a class that has a function for connection and you just pass the instance of that class to every class where you need the connection (here the db instance), or may be not!
It's just an idea that I used in a other languages.
A few options:
Implement a save/load method for each DataPoint child to build the select/insert/update sql commands according with the needs of each class. Here is a basic tutorial: Python MySQL Database Access
Implement a class to build the sql commands above automatically from your class attributes.
The above is called Object Relational Mapping and, so, you don't need to reinvent the wheel: https://www.fullstackpython.com/object-relational-mappers-orms.html
I'd go with the first if it is a simple project with no chance to become big and with an ORM tool for all the other cases.
The most popular for python is SQLAlchemy:
http://docs.sqlalchemy.org/en/latest/orm/tutorial.html
I have my own class Article designed to wrok with PostgreSQL. Each object created from the class is used to work with one row. Now I dont know how to test exception case. When I create such case:
article = Article(2)/*connects to the db and loads line with id 2*/
print article.title2 /*here my db does not have table title2 and should launch an error*/
it should throw error. And it does)
How should test case looks like? I use unittest. My test class with my wrong method which does not work is below:
import unittest
from article import * /*import my ORM class*/
class EntityTest(unittest.TestCase):
def setUp(self):
Entity.db = psycopg2.connect("dbname='postgres' user='postgres' host='192.168.136.129' password='xxxxxxxxx'")/*creates connection to db*/
def test_should_lounch_attr_error(self):
article = Article(2)
print article.title2
self.assertRaisesRegex(article.AttributeError, "No attribute exists")
I have no expirience how to create test cases and no much good documentation of how to do it(((
As I undestood if testcase is passed (exception is generated), unittest should return Ok statement. Now it just shows error.
If you do not supply a callable to assertRaisesRegexp() (N.B. assertRaisesRegexp(), not assertRaisesRegex()), then it acts as a context manager. In that case you should use a with statement like this:
import unittest
from article import * /*import my ORM class*/
class EntityTest(unittest.TestCase):
def setUp(self):
Entity.db = psycopg2.connect("dbname='postgres' user='postgres' host='192.168.136.129' password='xxxxxxxxx'")/*creates connection to db*/
def test_should_lounch_attr_error(self):
with self.assertRaisesRegexp(article.AttributeError, "No attribute exists"):
article = Article(2)
print article.title2
Unless your code can raise article.AttributeError with different string representations, I don't think that you actually need to use a regex for this. Just check for article.AttributeError with assertRaises(). This should suffice:
with self.assertRaisesRegexp(article.AttributeError):
article = Article(2)
print article.title2
I have problem with using class instance in Python.
Ive created a new class ora which inherit connect class from cx_Oracle package.
When I try tu run this code I recive information
File "pyt.py", line 12, in myquery
ora.myConnect.cursor()
AttributeError: 'NoneType' object has no attribute 'cursor'
So Python cannote recognize that in ora.myConnect is stored reference to instance.
I dont know what can be reason of this error and what its wrong with code.
from cx_Oracle import connect
class ora(connect):
myConnect = None
def __init__(self,connstr):
ora.myConnect = connect.__init__(self,connstr)
def myquery(self):
ora.myConnect.cursor()
ora.myConnect.cursor.execute("SELECT * FROM table")
ora.myConnect.cursor.close()
connstr = 'user/passwd#host:port/sid'
connection = ora(connstr)
connection.myquery()
connection.close()
EDIT
Ive tried to replace ora to self but still Python dont have access to instance
from cx_Oracle import connect
class ora(connect):
myConnect = None
def __init__(self,connstr):
self.myConnect = connect.__init__(self,connstr)
def myquery(self):
self.myConnect.cursor()
self.myConnect.cursor.execute("SELECT * FROM table")
self.myConnect.cursor.close()
Error:
self.myConnect.cursor()
AttributeError: 'NoneType' object has no attribute 'cursor'
EDIT2
This code works without OOP, for me self.myConnect sholud reference to object instance and this object should contain method cursor()
import cx_oracle
connstr = 'user/passwd#host:port/sid'
connection = cx_oracle.connect(connstr)
cursor = connection.cursor()
cursor.execute("SELECT * FROM table")
cursor.close()
connection.close()
It seems like you want self:
class ora(connect):
myConnect = None
def __init__(self, connstr):
self.myConnect = connect.__init__(self, connstr)
# ...
ora is the name of the class, not the instance.
Update Try the following:
from cx_Oracle import connect
class ora:
myConnect = None
def __init__(self, connstr):
self.myConnect = connect(connstr)
def myquery(self):
self.myConnect.cursor()
self.myConnect.cursor.execute("SELECT * FROM table")
self.myConnect.cursor.close()
Why do you want self.myConnect to refer to the connect instance? That's a complete misunderstanding of OOP. The ora instance is the connect instance. self.cursor is where you find the cursor.
Here's how your code should look:
class ora(connect):
def __init__(self,connstr):
super(ora, self).__init__(connstr)
def myquery(self):
self.cursor.execute("SELECT * FROM table")
self.cursor.close()
In any case, __init__ must never return anything, so setting self.myConnect to the return value will always result in it being bound to None.