Python - Classes and OOP Basics
Question: I do not fully understand classes. I have read the python documentation and
several other tutorials. I get the basic gist of it but don't understand the
nuance. For instance in my code here:

    
    
    class whiteroom():
        """ Pick a door: red, blue, green, or black. """
    
        do = raw_input("> ")
    
        if "red" in do:
            print "You entered the red room."
    
        elif "blue" in do:
            print "You entered the blue room."
    
        elif "green" in do:
            print "You entered the green room."
    
        elif "black" in do:
            print "You entered the black room."
    
        else:
            print "You sit patiently but slowly begin to stave.  You're running out of time."
            return whiteroom()
    
    game = whiteroom()
    game
    

(original [codepad](http://codepad.org/IP8IpfOl))

I would like to return the class whiteroom. Which is, either not possible, or
not being done correctly. If you could clear up how to return a class or how
to "link" two classes together so that whiteroom repeats on the else and the
other rooms (which would be classes) are returned when called that would be
awesome.

Also I'm super shaky on `__init__` and am still not really sure what its
purpose is. Everyone keeps telling me that it "initializes", which I'm sure it
does, but that doesn't seem to be helping my brain out.


Answer: Functions are very different from classes. It looks like you took a function
and just changed the `def` to `class`. I guess that _mostly_ works in your
case, but it's not how classes are supposed to go.

Classes contain functions (methods) and data. For example, you have a ball:

    
    
    class Ball(object):
        # __init__ is a special method called whenever you try to make
        # an instance of a class. As you heard, it initializes the object.
        # Here, we'll initialize some of the data.
        def __init__(self):
            # Let's add some data to the [instance of the] class.
            self.position = (100, 100)
            self.velocity = (0, 0)
    
        # We can also add our own functions. When our ball bounces,
        # its vertical velocity will be negated. (no gravity here!)
        def bounce(self):
            self.velocity = (self.velocity[0], -self.velocity[1])
    

Now we have a `Ball` class. How can we use it?

    
    
    >>> ball1 = Ball()
    >>> ball1
    <Ball object at ...>
    

It doesn't look very useful. The data is where it could be useful:

    
    
    >>> ball1.position
    (100, 100)
    >>> ball1.velocity
    (0, 0)
    >>> ball1.position = (200, 100)
    >>> ball1.position
    (200, 100)
    

Alright, cool, but what's the advantage over a global variable? If you have
another `Ball` instance, it will remain independent:

    
    
    >>> ball2 = Ball()
    >>> ball2.velocity = (5, 10)
    >>> ball2.position
    (100, 100)
    >>> ball2.velocity
    (5, 10)
    

And `ball1` remains independent:

    
    
    >>> ball1.velocity
    (0, 0)
    

Now what about that `bounce` method (function in a class) we defined?

    
    
    >>> ball2.bounce()
    >>> ball2.velocity
    (5, -10)
    

The `bounce` method caused it to modify the `velocity` data of itself. Again,
`ball1` was not touched:

    
    
    >>> ball1.velocity
    

# Application

A ball is neat and all, but most people aren't simulating that. You're making
a game. Let's think of what kinds of things we have:

  * **A room** is the most obvious thing we could have.

So let's make a room. Rooms have names, so we'll have some data to store that:

    
    
    class Room(object):
        # Note that we're taking an argument besides self, here.
        def __init__(self, name):
            self.name = name  # Set the room's name to the name we got.
    

And let's make an instance of it:

    
    
    >>> white_room = Room("White Room")
    >>> white_room.name
    'White Room'
    

Spiffy. This turns out not to be all that useful if you want different rooms
to have different functionality, though, so let's make a _subclass_. A
_subclass_ inherits all functionality from its _superclass_ , but you can add
more functionality or override the superclass's functionality.

Let's think about what we want to do with rooms:

> We want to interact with rooms.

And how do we do that?

> The user types in a line of text that gets responded to.

How it's responded do depends on the room, so let's make the room handle that
with a method called `interact`:

    
    
    class WhiteRoom(Room):  # A white room is a kind of room.
        def __init__(self):
            # All white rooms have names of 'White Room'.
            self.name = 'White Room'
    
        def interact(self, line):
            if 'test' in line:
                print "'Test' to you, too!"
    

Now let's try interacting with it:

    
    
    >>> white_room = WhiteRoom()  # WhiteRoom's __init__ doesn't take an argument (even though its superclass's __init__ does; we overrode the superclass's __init__)
    >>> white_room.interact('test')
    'Test' to you, too!
    

Your original example featured moving between rooms. Let's use a global
variable called `current_room` to track which room we're in.1 Let's also make
a red room.

1\. There's better options besides global variables here, but I'm going to use
one for simplicity.

    
    
    class RedRoom(Room):  # A red room is also a kind of room.
        def __init__(self):
            self.name = 'Red Room'
    
        def interact(self, line):
            global current_room, white_room
            if 'white' in line:
                # We could create a new WhiteRoom, but then it
                # would lose its data (if it had any) after moving
                # out of it and into it again.
                current_room = white_room
    

Now let's try that:

    
    
    >>> red_room = RedRoom()
    >>> current_room = red_room
    >>> current_room.name
    'Red Room'
    >>> current_room.interact('go to white room')
    >>> current_room.name
    'White Room'
    

**Exercise for the reader:** Add code to `WhiteRoom`'s `interact` that allows
you to go back to the red room.

Now that we have everything working, let's put it all together. With our new
`name` data on all rooms, we can also show the current room in the prompt!

    
    
    def play_game():
        global current_room
        while True:
            line = raw_input(current_room.name + '> ')
            current_room.interact(line)
    

You might also want to make a function to reset the game:

    
    
    def reset_game():
        global current_room, white_room, red_room
        white_room = WhiteRoom()
        red_room = RedRoom()
        current_room = white_room
    

Put all of the class definitions and these functions into a file and you can
play it at the prompt like this (assuming they're in `mygame.py`):

    
    
    >>> import mygame
    >>> mygame.reset_game()
    >>> mygame.play_game()
    White Room> test
    'Test' to you, too!
    White Room> go to red room
    Red Room> go to white room
    White Room>
    

To be able to play the game just by running the Python script, you can add
this at the bottom:

    
    
    def main():
        reset_game()
        play_game()
    
    if __name__ == '__main__':  # If we're running as a script...
        main()
    

And that's a basic introduction to classes and how to apply it to your
situation.

