Python singleton database connection

DB-Connections Class as a Singleton in Python

Normally, you have some kind of object representing the thing that uses a database (e.g., an instance of MyWebServer ), and you make the database connection a member of that object.

If you instead have all your logic inside some kind of function, make the connection local to that function. (This isn’t too common in many other languages, but in Python, there are often good ways to wrap up multi-stage stateful work in a single generator function.)

If you have all the database stuff spread out all over the place, then just use a global variable instead of a singleton. Yes, globals are bad, but singletons are just as bad, and more complicated. There are a few cases where they’re useful, but very rare. (That’s not necessarily true for other languages, but it is for Python.) And the way to get rid of the global is to rethink you design. There’s a good chance you’re effectively using a module as a (singleton) object, and if you think it through, you can probably come up with a good class or function to wrap it up in.

Obviously just moving all of your globals into class attributes and @classmethod s is just giving you globals under a different namespace. But moving them into instance attributes and methods is a different story. That gives you an object you can pass around—and, if necessary, an object you can have 2 of (or maybe even 0 under some circumstances), attach a lock to, serialize, etc.

In many types of applications, you’re still going to end up with a single instance of something—every Qt GUI app has exactly one MyQApplication , nearly every web server has exactly one MyWebServer , etc. No matter what you call it, that’s effectively a singleton or global. And if you want to, you can just move everything into attributes of that god object.

But just because you can do so doesn’t mean you should. You’ve still got function parameters, local variables, globals in each module, other (non-megalithic) classes with their own instance attributes, etc., and you should use whatever is appropriate for each value.

For example, say your MyWebServer creates a new ClientConnection instance for each new client that connects to you. You could make the connections write MyWebServer.instance.db.execute whenever they want to execute a SQL query… but you could also just pass self.db to the ClientConnection constructor, and each connection then just does self.db.execute . So, which one is better? Well, if you do it the latter way, it makes your code a lot easier to extend and refactor. If you want to load-balance across 4 databases, you only need to change code in one place (where the MyWebServer initializes each ClientConnection ) instead of 100 (every time the ClientConnection accesses the database). If you want to convert your monolithic web app into a WSGI container, you don’t have to change any of the ClientConnection code except maybe the constructor. And so on.

If you’re using an object oriented approach, then abamet’s suggestion of attaching the database connection parameters as class attributes makes sense to me. The class can then establish a single database connection which all methods of the class refer to as self.db_connection , for example.

If you’re not using an object oriented approach, a separate database connection module can provide a functional-style equivalent. Devote a module to establishing a database connection, and simply import that module everywhere you want to use it. Your code can then refer to the connection as db.connection , for example. Since modules are effectively singletons, and the module code is only run on the first import, you will be re-using the same database connection each time.

Jeffrey Froman 5643

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Источник

How to do singleton database connection in python

Amazon DynamoDB(Beta) – is a fully managed NoSQL database service that provides fast and predictable performance with seamless scalability.

And I use boto , its a Python package that provides interfaces to Amazon Web Services

class DbConnection create a singleton database connection and DynomoDbOperations does the basic dynamodb operations.

following is a code sample , to make a singleton connection with dynamodb

#make sure boto installed
from boto import connect_dynamodb
from boto.exception import BotoClientError,DynamoDBResponseError

class DbConnection(object):

_instance = None

# inner class
class Singleton:
def __init__(self):
# singleton variable

#configure boto or supply necessary parameters with connect_dynamodb
self.connection = connect_dynamodb()
# constructor of DbConnection
def __init__( self):

# check whether instace is already created or not
if DbConnection._instance is None:
DbConnection._instance = DbConnection.Singleton()
self._EventHandler_instance = DbConnection._instance

def __getattr__(self, aAttr):
return getattr(self._instance, aAttr)
def __setattr__(self, aAttr, aValue):
return setattr(self._instance, aAttr, aValue)

# Follwing class does basic dynamodb operations get,put,update,delete
class DynamoDbOperations():
TABLE_NAME = ‘tbltest’
# method getting connection with DynamoDb

def get_dynamodb_connection(self):

self.dynamodb_connection = DbConnection().connection

# printing the connection object for testing

print self.dynamodb_connection

return self.dynamodb_connection

except BotoClientError as exe:

def get_dynamodb_table(self):

return self.get_dynamodb_connection().get_table(self.TABLE_NAME)

except DynamoDBResponseError as exe:

# method put values to dynamodb

def put_dynanodb(self , attr_dict):

self.get_dynamodb_table().new_item(hash_key = None ,range_key = None,attrs = attr_dict ).put()

except BotoClientError as exe:

# method getting values from Dynamodb using the unique key

def get_dynamodb(self, key):

return self.get_dynamodb_table().get_item(key)
except BotoClientError as exe:

# method updating DynamoDb

def update_dynamodb(self , key , additional_items_dict):

item = self.get_dynamodb(key)
for key in additional_items_dict.iterkeys():

itemPython singleton database connection = additional_items_dictPython singleton database connection
item.put()

except BotoClientError as exe:

# metod delete from DynamoDb

def delete_from_dynamodb(self, key):

self.get_dynamodb(key).delete()

except BotoClientError as exe:

Test it by cerating two different objects of DynamoDbOperations and calling the method get_dynamodb, both object make use of the same database connection ,we can verify it by examining the objects printed in the method get_dynamodb_connection ,both points to the same address location.

print DynamoDbOperations().get_dynamodb(‘abc’)
print DynamoDbOperations().get_dynamodb(‘def’)

Источник