Categories
python

Example for singleton decorator pattern in python

I know there is not very common in Python to use the singleton pattern, but I found a nice implementation of this pattern in Python 3 Patterns, Recipes and Idioms book. Starting with that example I implemented an equivalent of the well known PHP getConnection example.

You have the code below:

This is the class that implements the Singleton pattern.

[code language=”python”]
class Singleton:
def __init__(self, klass):
self.klass = klass
self.instance = None
def __call__(self, *args, **kwds):
if self.instance == None:
self.instance = self.klass(*args, **kwds)
return self.instance
[/code]

Now, we create a class and we decorate it with the Singleton class. Let’s import also MySQLdb module*.

[code language=”python”]
import MySQLdb

@Singleton
class Database:
connection = None
def get_connection(self):
if self.connection is None:
self.connection = MySQLdb.connect(host="localhost", user="root", passwd="razvan", db="mydatabase")
return self.connection
[/code]

Let’s test this:

[code language=”python”]
db1 = Database().get_connection()
db2 = Database().get_connection()

print (db2)
print (db1)
[/code]

You will see something like:

[code language=”bash”]
<_mysql.connection open to ‘localhost’ at 16b4800>
<_mysql.connection open to ‘localhost’ at 16b4800>
[/code]

As you can see there is only one object.

For fun, let’s remove the line “@Singleton” and re-run the example. This time you will see different objects:

[code language=”bash”]
<_mysql.connection open to ‘localhost’ at c91e20>
<_mysql.connection open to ‘localhost’ at bccba0>
[/code]

You can find the fully example here.

* If you don’t know how to install MySQLdb, you can check our previous post.

Categories
Programming python

Install MySQLdb for Python 2.x

If you want to install MySQLdb for Python 2.x, without using the package manager of your Linux distro (Ubuntu in our case), you can do it through pip like this:

First you must to have installed libmysqlclient-dev (and, of course pip)

[source language=”bash”]
sudo apt-get install libmysqlclient-dev
[/source]

After that execute this two commands:

[source language=”bash”]
sudo easy_install -U distribute
sudo pip install mysql-python
[/source]

Test if is working:
Type in the command line:

[source language=”bash”]
python
[/source]

and in the python console:

[source language=”python”]
>>> import MySQLdb
>>> help(MySQLdb)
[/source]

That’s all. Enjoy!

Categories
Programming

Tip: List vs. Set in Python

In Python, if you have a “list” with a lot of members and you need to do many membership checks, it will be better to use a “set” instead, because the checks are linear for lists and constant for sets.