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Python: single instance of program

Is there a Pythonic way to have only one instance of a program running?

The only reasonable solution I've come up with is trying to run it as a server on some port, then second program trying to bind to same port - fails. But it's not really a great idea, maybe there's something more lightweight than this?

(Take into consideration that program is expected to fail sometimes, i.e. segfault - so things like "lock file" won't work)

Update: the solutions offered are much more complex and less reliant than just having a port occupied with a non-existent server, so I'd have to go with that one.

What's Alternative to Singleton

We have a class that holds configuration information for the application. It used to be a singleton. After some architectural review, we were told to remove the singleton. We did see some benefits of not using singleton in the unit testing because we can test different configurations all at once.

Without singleton, we have to pass the instance around everywhere in our code. It's getting so messy so we wrote a singleton wrapper. Now we are porting the same code to PHP and .NET, I am wondering if there is a better pattern we can use for the configuration object.

How to declare global variables in Android?

I am creating an application which requires login. I created the main and the login activity.

In the main activity onCreate method I added the following condition:

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    ...

    loadSettings();
    if(strSessionString == null)
    {
        login();
    }
    ...
}

The onActivityResult method which is executed when the login form terminates looks like this:

@Override
public void onActivityResult(int requestCode,
                             int resultCode,
                             Intent data)
{
    super.onActivityResult(requestCode, resultCode, data);
    switch(requestCode)
    {
        case(SHOW_SUBACTICITY_LOGIN):
        {
            if(resultCode == Activity.RESULT_OK)
            {

                strSessionString = data.getStringExtra(Login.SESSIONSTRING);
                connectionAvailable = true;
                strUsername = data.getStringExtra(Login.USERNAME);
            }
        }
    }

The problem is the login form sometimes appears twice (the login() method is called twice) and also when the phone keyboard slides the login form appears again and I guess the problem is the variable strSessionString.

Does anyone know how to set the variable global in order to avoid login form appearing after the user already successfully authenticates?

Thanks!

What is so bad about singletons? [closed]

The singleton pattern is a fully paid up member of the GoF's patterns book, but it lately seems rather orphaned by the developer world. I still use quite a lot of singletons, especially for factory classes, and while you have to be a bit careful about multithreading issues (like any class actually), I fail to see why they are so awful.

Stack Overflow especially seems to assume that everyone agrees that Singletons are evil. Why?

What is an efficient way to implement a singleton pattern in Java?

What is an efficient way to implement a singleton pattern in Java?

class << self idiom in Ruby

What does class << self do in Ruby?

Difference between static class and singleton pattern?

What real (i.e. practical) difference exists between a static class and a singleton pattern?

Both can be invoked without instantiation, both provide only one "Instance" and neither of them is thread-safe. Is there any other difference?

What is a singleton in C#?

Pretty straight forward question.

What is a Singleton and when should I use it?

Javascript: best Singleton pattern [duplicate]

Possible Duplicate:
Simplest/Cleanest way to implement singleton in JavaScript?

I'm using this pattern for singletons, in the example the singleton is PlanetEarth:

var NAMESPACE = function () {

    var privateFunction1 = function () {
        privateFunction2();
    };

    var privateFunction2 = function () {
        alert('I\'m private!');
    };

    var Constructors = {};

    Constructors.PlanetEarth = function () {
        privateFunction1();
        privateFunction2();
    };

    Constructors.PlanetEarth.prototype = {
        someMethod: function () {
            if (console && console.log) {
                console.log('some method');             
            }
        }
    };

    Constructors.Person = function (name, address) {
        this.name = name;
        this.address = address;
    };

    Constructors.Person.prototype = {
        walk: function () {
            alert('STOMP!');
        }
    };

    return {
        Person: Constructors.Person, // there can be many
        PlanetEarth: new Constructors.PlanetEarth() // there can only be one!
    };

}();

Since PlanetEarth's constructor remains private, there can only be one.

Now, something tells me that this self-cooked thing isn't the best one can do, mostly because I don't have an academic education and I tend to solve problems in stupid ways. What would you propose as a better alternative my method, where better is defined as stylistically better and/or more powerful?

Are there any viable alternatives to the GOF Singleton Pattern?

Let's face it. The Singleton Pattern is highly controversial topic with hordes programmers on both sides of the fence. There are those who feel like the Singleton is nothing more then a glorified global variable, and others who swear by pattern and use it incessantly. I don't want the Singleton Controversy to lie at the heart of my question, however. Everyone can have a tug-of-war and battle it out and see who wins for all I care. What I'm trying to say is, I don't believe there is a single correct answer and I'm not intentionally trying inflame partisan bickering. I am simply interested in singleton-alternatives when I ask the question:

Are their any specific alternatives to the GOF Singleton Pattern?

For example, many times when I have used the singleton pattern in the past, I am simply interested in preserving the state/values of one or several variables. The state/values of variables, however, can be preserved between each instantiation of the class using static variables instead of using the singleton pattern.

What other idea's do you have?

EDIT: I don't really want this to be another post about "how to use the singleton correctly." Again, I'm looking for ways to avoid it. For fun, ok? I guess I'm asking a purely academic question in your best movie trailer voice, "In a parallel universe where there is no singleton, what could we do?"

Is there a use-case for singletons with database access in PHP?

I access my MySQL database via PDO. I'm setting up access to the database, and my first attempt was to use the following:

The first thing I thought of is global:

$db = new PDO('mysql:host=127.0.0.1;dbname=toto', 'root', 'pwd');

function some_function() {
    global $db;
    $db->query('...');
}

This is considered a bad practice. After a little search, I ended up with the Singleton pattern, which

"applies to situations in which there needs to be a single instance of a class."

According to the example in the manual, we should do this:

class Database {
    private static $instance, $db;

    private function __construct(){}

    static function singleton() {
        if(!isset(self::$instance))
            self::$instance = new __CLASS__;

        return self:$instance;
    }

    function get() {
        if(!isset(self::$db))
            self::$db = new PDO('mysql:host=127.0.0.1;dbname=toto', 'user', 'pwd')

        return self::$db;
    }
}

function some_function() {
    $db = Database::singleton();
    $db->get()->query('...');
}

some_function();

Why do I need that relatively large class when I can do this?

class Database {
    private static $db;

    private function __construct(){}

    static function get() {
        if(!isset(self::$rand))
            self::$db = new PDO('mysql:host=127.0.0.1;dbname=toto', 'user', 'pwd');

        return self::$db;
    }
}

function some_function() {
    Database::get()->query('...');
}

some_function();

This last one works perfectly and I don't need to worry about $db anymore.

How can I create a smaller singleton class, or is there a use-case for singletons that I'm missing in PHP?

How to create module-wide variables in Python?

Is there a way to set up a global variable inside of a module? When I tried to do it the most obvious way as appears below, the Python interpreter said the variable __DBNAME__ did not exist.

...
__DBNAME__ = None

def initDB(name):
    if not __DBNAME__:
        __DBNAME__ = name
    else:
        raise RuntimeError("Database name has already been set.")
...

And after importing the module in a different file

...
import mymodule
mymodule.initDB('mydb.sqlite')
...

And the traceback was: UnboundLocalError: local variable '__DBNAME__' referenced before assignment

Any ideas? I'm trying to set up a singleton by using a module, as per this fellow's recommendation.

Singleton by Jon Skeet clarification

public sealed class Singleton
{
    Singleton()
    {
    }

    public static Singleton Instance
    {
        get
        {
            return Nested.instance;
        }
    }

    class Nested
    {
        // Explicit static constructor to tell C# compiler
        // not to mark type as beforefieldinit
        static Nested()
        {
        }

        internal static readonly Singleton instance = new Singleton();
    }
}

I wish to implement Jon Skeet's Singleton pattern in my current application in C#.

I have two doubts on the code

1. How is it possible to access the outer class inside nested class? I mean

   internal static readonly Singleton instance = new Singleton();
   

   Is something called closure?

2. I did not get this comment

   // Explicit static constructor to tell C# compiler
   // not to mark type as beforefieldinit
   

   what does this comment suggest us?

Singleton: How should it be used

Edit: From another question I provided an answer that has links to a lot of questions/answers about singletons: More info about singletons here:

So I have read the thread Singletons: good design or a crutch?
And the argument still rages.

I see Singletons as a Design Pattern (good and bad).

The problem with Singleton is not the Pattern but rather the users (sorry everybody). Everybody and their father thinks they can implement one correctly (and from the many interviews I have done, most people can't). Also because everybody thinks they can implement a correct Singleton they abuse the Pattern and use it in situations that are not appropriate (replacing global variables with Singletons!).

So the main questions that need to be answered are:

• When should you use a Singleton
• How do you implement a Singleton correctly

My hope for this article is that we can collect together in a single place (rather than having to google and search multiple sites) an authoritative source of when (and then how) to use a Singleton correctly. Also appropriate would be a list of Anti-Usages and common bad implementations explaining why they fail to work and for good implementations their weaknesses.

So get the ball rolling:
I will hold my hand up and say this is what I use but probably has problems.
I like "Scott Myers" handling of the subject in his books "Effective C++"

Good Situations to use Singletons (not many):

• Logging frameworks
• Thread recycling pools

/*
 * C++ Singleton
 * Limitation: Single Threaded Design
 * See: http://www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf
 *      For problems associated with locking in multi threaded applications
 *
 * Limitation:
 * If you use this Singleton (A) within a destructor of another Singleton (B)
 * This Singleton (A) must be fully constructed before the constructor of (B)
 * is called.
 */
class MySingleton
{
    private:
        // Private Constructor
        MySingleton();
        // Stop the compiler generating methods of copy the object
        MySingleton(MySingleton const& copy);            // Not Implemented
        MySingleton& operator=(MySingleton const& copy); // Not Implemented

    public:
        static MySingleton& getInstance()
        {
            // The only instance
            // Guaranteed to be lazy initialized
            // Guaranteed that it will be destroyed correctly
            static MySingleton instance;
            return instance;
        }
};

OK. Lets get some criticism and other implementations together.
:-)

Python and the Singleton Pattern [duplicate]

Possible Duplicate:
Is there a simple, elegant way to define Singletons in Python?

What is the best way to implement the singleton pattern in Python? It seems impossible to declare the constructor private or protected as is normally done with the Singleton pattern...