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 Help with classes
#1
Hi All,

I'm sure this forum is littered with Learn Python the Hard Way questions... here's another on exercise 43: learnpythonthehardway.org/book/ex43.html. I've just started learning (without any programming background), so please excuse the simplicity of the questions.

Essentially, I'm trying to wrap my head around how everything "works" systematically and in what order. The code for the game is at the link above, but I've also pasted it at the end of this message. My questions (to start with) are:

If a class is called (e.g. the map class at the end), are all methods\functions run within this class then and there? Or, is only the __init__ method run and the subsequent functions run when play() is called?

So, with the following line:    
a_map = Map('central_corridor')

Does this define 'central_corridor' as the 'start_scene' and stop there? Or, does it carry on to the 'opening_scene()' and run 'next_scene'?

Can someone please list in order what occurs with the following lines of code:
a_map = Map('central_corridor')
a_game = Engine(a_map)
a_game.play()
Thanks a lot!!!_______________________________________________________________________________________________________

Full script:
from sys import exit

from random import randint

class Scene(object):

    def enter(self):
        print """This scene is not yet configured. Subclass it and implement
        enter()."""
        exit(1)

class Engine(object):

    def __init__(self, scene_map):
        self.scene_map = scene_map

    def play(self):
        current_scene = self.scene_map.opening_scene()
        last_scene = self.scene_map.next_scene('finished')

        while current_scene != last_scene:
            next_scene_name = current_scene.enter()
            current_scene = self.scene_map.next_scene(next_scene_name)

        # be sure to print out the last scene
        current_scene.enter()

class Death(Scene):

    quips = [
        "You died. You kinda suck at this.",
        "Your mom would be proud... if she were smarter.",
        "Such a loser.",
        "I have a small puppy that's better at this."
    ]

    def enter(self):
        print Death.quips[randint(0, len(self.quips)-1)]
        exit(1)

class CentralCorridor(Scene):

    def enter(self):
        print "The Gothons of Planet Percal #25 have invaded your ship and destroyed"
        print "your entire crew.  You are the last surviving member and your last"
        print "mission is to get the neutron destruct bomb from the Weapons Armory,"
        print "put it in the bridge, and blow the ship up after getting into an "
        print "escape pod."
        print "\n"
        print "You're running down the central corridor to the Weapons Armory when"
        print "a Gothon jumps out, red scaly skin, dark grimy teeth, and evil clown costume"
        print "flowing around his hate filled body.  He's blocking the door to the"
        print "Armory and about to pull a weapon to blast you."

        action = raw_input("> ")

        if action == "shoot!":
            print "Quick on the draw you yank out your blaster and fire it at the Gothon."
            print "His clown costume is flowing and moving around his body, which throws"
            print "off your aim.  Your laser hits his costume but misses him entirely.  This"
            print "completely ruins his brand new costume his mother bought him, which"
            print "makes him fly into an insane rage and blast you repeatedly in the face until"
            print "you are dead.  Then he eats you."
            return 'death'

        elif action == "dodge!":
            print "Like a world class boxer you dodge, weave, slip and slide right"
            print "as the Gothon's blaster cranks a laser past your head."
            print "In the middle of your artful dodge your foot slips and you"
            print "bang your head on the metal wall and pass out."
            print "You wake up shortly after only to die as the Gothon stomps on"
            print "your head and eats you."
            return 'death'

        elif action == "tell a joke":
            print "Lucky for you they made you learn Gothon insults in the academy."
            print "You tell the one Gothon joke you know:"
            print "Lbhe zbgure vf fb sng, jura fur fvgf nebhaq gur ubhfr, fur fvgf nebhaq gur ubhfr."
            print "The Gothon stops, tries not to laugh, then busts out laughing and can't move."
            print "While he's laughing you run up and shoot him square in the head"
            print "putting him down, then jump through the Weapon Armory door."
            return 'laser_weapon_armory'

        else:
            print "DOES NOT COMPUTE!"
            return 'central_corridor'

class LaserWeaponArmory(Scene):

    def enter(self):
        print "You do a dive roll into the Weapon Armory, crouch and scan the room"
        print "for more Gothons that might be hiding.  It's dead quiet, too quiet."
        print "You stand up and run to the far side of the room and find the"
        print "neutron bomb in its container.  There's a keypad lock on the box"
        print "and you need the code to get the bomb out.  If you get the code"
        print "wrong 10 times then the lock closes forever and you can't"
        print "get the bomb.  The code is 3 digits."
        code = "%d%d%d" % (randint(1,9), randint(1,9), randint(1,9))
        guess = raw_input("[keypad]> ")
        guesses = 0

        while guess != code and guesses < 10:
            print "BZZZEDDD!"
            guesses += 1
            guess = raw_input("[keypad]> ")

        if guess == code:
            print "The container clicks open and the seal breaks, letting gas out."
            print "You grab the neutron bomb and run as fast as you can to the"
            print "bridge where you must place it in the right spot."
            return 'the_bridge'

        else:
            print "The lock buzzes one last time and then you hear a sickening"
            print "melting sound as the mechanism is fused together."
            print "You decide to sit there, and finally the Gothons blow up the"
            print "ship from their ship and you die."
            return 'death'

class TheBridge(Scene):

    def enter(self):
        print "You burst onto the Bridge with the netron destruct bomb"
        print "under your arm and surprise 5 Gothons who are trying to"
        print "take control of the ship.  Each of them has an even uglier"
        print "clown costume than the last.  They haven't pulled their"
        print "weapons out yet, as they see the active bomb under your"
        print "arm and don't want to set it off."

        action = raw_input("> ")

        if action == "throw the bomb":
            print "In a panic you throw the bomb at the group of Gothons"
            print "and make a leap for the door.  Right as you drop it a"
            print "Gothon shoots you right in the back killing you."
            print "As you die you see another Gothon frantically try to disarm"
            print "the bomb. You die knowing they will probably blow up when"
            print "it goes off."
            return 'death'

        elif action == "slowly place the bomb":
            print "You point your blaster at the bomb under your arm"
            print "and the Gothons put their hands up and start to sweat."
            print "You inch backward to the door, open it, and then carefully"
            print "place the bomb on the floor, pointing your blaster at it."
            print "You then jump back through the door, punch the close button"
            print "and blast the lock so the Gothons can't get out."
            print "Now that the bomb is placed you run to the escape pod to"
            print "get off this tin can."
            return 'escape_pod'

        else:
            print "DOES NOT COMPUTE!"
            return "the_bridge"

class EscapePod(Scene):

    def enter(self):
        print "You rush through the ship desperately trying to make it to"
        print "the escape pod before the whole ship explodes.  It seems like"
        print "hardly any Gothons are on the ship, so your run is clear of"
        print "interference.  You get to the chamber with the escape pods, and"
        print "now need to pick one to take.  Some of them could be damaged"
        print "but you don't have time to look.  There's 5 pods, which one"
        print "do you take?"

        good_pod = randint(1,5)
        guess = raw_input("[pod #]> ")


        if int(guess) != good_pod:
            print "You jump into pod %s and hit the eject button." % guess
            print "The pod escapes out into the void of space, then"
            print "implodes as the hull ruptures, crushing your body"
            print "into jam jelly."
            return 'death'

        else:
            print "You jump into pod %s and hit the eject button." % guess
            print "The pod easily slides out into space heading to"
            print "the planet below.  As it flies to the planet, you look"
            print "back and see your ship implode then explode like a"
            print "bright star, taking out the Gothon ship at the same"
            print "time.  You won!"

            return 'finished'

class Finished(Scene):

    def enter(self):
        print "You won! Good job."
        return 'finished'

class Map(object):

    scenes = {
        'central_corridor': CentralCorridor(),
        'laser_weapon_armory': LaserWeaponArmory(),
        'the_bridge': TheBridge(),
        'escape_pod': EscapePod(),
        'death': Death(),
        'finished': Finished(),
    }

    def __init__(self, start_scene):
        self.start_scene = start_scene

    def next_scene(self, scene_name):
        val = Map.scenes.get(scene_name)
        return val

    def opening_scene(self):
        return self.next_scene(self.start_scene)

a_map = Map('central_corridor')
a_game = Engine(a_map)
a_game.play()
Quote
#2
It's not exactly true that only the __init__ method of a class is called when an instance of a class is created. There are other things going on in the background, but you don't need to worry about them when you're starting out.

With the three lines of code:

a_map = Map('central_corridor')
a_game = Engine(a_map)
a_game.play()
The first line creates an instance of the Map class, and assigns 'central_corridor' to the start_scene attribute of that instance. The second line creates an instance of the Engine class, and assigns a_map to the map attribute of that instance. The third line calls the play method of the Engine instance created by the second line.

You might want to check out this tutorial on classes.
Craig "Ichabod" O'Brien - xenomind.com
I wish you happiness.
Recommended Tutorials: BBCode, functions, classes, text adventures

Quote
#3
Thanks ichabod801. I'll be sure to check out the tutorial.

Regarding my original question. You mentioned that not only the __init__ method is called. If we use the map class as an example, will the other 2 functions, 'next_scene' and 'opening_scene' be called by default along with the __init__method whenever an instance of map is created?
Quote
#4
No, they will not be called, unless you call them from within __init__. You can test this by adding something like print("method_name called") at the start of each of the functions, and you'll see in the console exactly when they're called.

That said, they are defined and attached to the new object, so each of your Map()s will have a different next_scene() method. ...although that rarely matters.
ichabod801 likes this post
Quote
#5
Great, thanks for clarifying nilamo.
nilamo likes this post
Quote
#6
Hi Guys, 

Wasn't sure whether to open a new thread, so continued on from my previous one (it's the same script). Please let me know if the best practice is to open a new one, and I will the next time.

So, I'm trying to change\add some features to the script, specifically adding new classes with their own functions. 

I'm getting the following error when trying to run the script below:
Error:
Traceback (most recent call last):   File "ex43_mine.py", line 254, in <module>     a_game.play()   File "ex43_mine.py", line 44, in play     next_scene_name = current_scene.enter()   File "ex43_mine.py", line 67, in enter     loaded_alien = load_alien('central_corridor') NameError: global name 'load_alien' is not defined
I can understand that the 'load_alien' is not defined anywhere, but I'm stuck on how to call it elegantly within the script. Essentially, I want "an alien" within the aliens dict to be retrieved for each corresponding scene. I'm probably making things more difficult for myself here, and the solution is straight forward. Can you point me in the right direction. Thank you.

 
from sys import exit
from random import randint

class Scene(object):

    def enter(self):
        print """This scene is not yet configured. Subclass it and implement
        enter()."""
        exit(1)

class Alien(object):

    def __init__(self, alien_name, alien_lair):
        self.alien_name = alien_name
        self.alien_lair = alien_lair
        self.health = 2

        aliens = {
            'central_corridor': 'puppeteer',
            'laser_weapon_armory': 'kzin',
            'the_bridge': 'trooper',
            'escape_pod': 'predator',
        }

    # loads corresponding alien onto scene
    def load_alien(alien_lair):
        alien = Alien.aliens.get(alien_lair)
        return alien


##attack or miss (use random)

class Engine(object):

    def __init__(self, scene_map):
        self.scene_map = scene_map

    def play(self):
        current_scene = self.scene_map.opening_scene()
        last_scene = self.scene_map.next_scene('finished')


        while current_scene != last_scene:
            next_scene_name = current_scene.enter()
            current_scene = self.scene_map.next_scene(next_scene_name)

        # be sure to print out the last scene
        current_scene.enter()

class Death(Scene):

    quips = [
        "You died. You kinda suck at this.",
        "Your mom would be proud... if she were smarter.",
        "Such a loser.",
        "I have a small puppy that's better at this."
    ]

    def enter(self):
        print Death.quips[randint(0, len(self.quips)-1)]
        exit(1)

class CentralCorridor(Scene):

    def enter(self):
        loaded_alien = load_alien('central_corridor')
        print "The Gothons of Planet Percal #25 have invaded your ship and destroyed"
        print "your entire crew.  You are the last surviving member and your last"
        print "mission is to get the neutron destruct bomb from the Weapons Armory,"
        print "put it in the bridge, and blow the ship up after getting into an "
        print "escape pod."
        print "\n"
        print "You're running down the central corridor to the Weapons Armory when"
        print "a %s high on crack jumps out, both heads bobbing to and fro" % loaded_alien,
        print "and yielding a laser beam. He's blocking the door to the"
        print "Armory and about to blast you. Quick, choose a weapon from your arsenal."

        weapon = raw_input("> ")

        if weapon == "feather":
            print "Quick on the draw you yank out your feather and tickle the Puppeteer"
            print "once he re-emerges from his perfectly curled sphere."

            print "you are dead.  Then he kicks you."
            return 'death'

        elif weapon != "feather":
            print "Sensing your move ahead of time, the higly intelligent Puppeteer adroitly"
            print "moves to the side, and you miss. While you are cursing your luck, the"
            print "little fucker ambushes you banshee-style forcing you to retreat."
            print "In your frenzy, you run headlong into a wall and lose consciousness."
            print "You wake up shortly after only to die as the Puppeteer stomps on"
            print "your head."
            return 'death'

        elif action == "tell a joke":
            print "Lucky for you they made you learn Gothon insults in the academy."
            print "You tell the one Gothon joke you know:"
            print "Lbhe zbgure vf fb sng, jura fur fvgf nebhaq gur ubhfr, fur fvgf nebhaq gur ubhfr."
            print "The Gothon stops, tries not to laugh, then busts out laughing and can't move."
            print "While he's laughing you run up and shoot him square in the head"
            print "putting him down, then jump through the Weapon Armory door."
            return 'laser_weapon_armory'

        else:
            print "DOES NOT COMPUTE!"
            return 'central_corridor'

class LaserWeaponArmory(Scene):

    def enter(self):
        print "You do a dive roll into the Weapon Armory, crouch and scan the room"
        print "for more Gothons that might be hiding.  It's dead quiet, too quiet."
        print "You stand up and run to the far side of the room and find the"
        print "neutron bomb in its container.  There's a keypad lock on the box"
        print "and you need the code to get the bomb out.  If you get the code"
        print "wrong 10 times then the lock closes forever and you can't"
        print "get the bomb.  The code is 3 digits."
        code = "%d%d%d" % (randint(1,9), randint(1,9), randint(1,9))
        guess = raw_input("[keypad]> ")
        guesses = 0

        while guess != code and guesses < 10:
            print "BZZZEDDD!"
            guesses += 1
            guess = raw_input("[keypad]> ")

        if guess == code:
            print "The container clicks open and the seal breaks, letting gas out."
            print "You grab the neutron bomb and run as fast as you can to the"
            print "bridge where you must place it in the right spot."
            return 'the_bridge'

        else:
            print "The lock buzzes one last time and then you hear a sickening"
            print "melting sound as the mechanism is fused together."
            print "You decide to sit there, and finally the Gothons blow up the"
            print "ship from their ship and you die."
            return 'death'

class TheBridge(Scene):

    def enter(self):
        print "You burst onto the Bridge with the netron destruct bomb"
        print "under your arm and surprise 5 Gothons who are trying to"
        print "take control of the ship.  Each of them has an even uglier"
        print "clown costume than the last.  They haven't pulled their"
        print "weapons out yet, as they see the active bomb under your"
        print "arm and don't want to set it off."

        action = raw_input("> ")

        if action == "throw the bomb":
            print "In a panic you throw the bomb at the group of Gothons"
            print "and make a leap for the door.  Right as you drop it a"
            print "Gothon shoots you right in the back killing you."
            print "As you die you see another Gothon frantically try to disarm"
            print "the bomb. You die knowing they will probably blow up when"
            print "it goes off."
            return 'death'

        elif action == "slowly place the bomb":
            print "You point your blaster at the bomb under your arm"
            print "and the Gothons put their hands up and start to sweat."
            print "You inch backward to the door, open it, and then carefully"
            print "place the bomb on the floor, pointing your blaster at it."
            print "You then jump back through the door, punch the close button"
            print "and blast the lock so the Gothons can't get out."
            print "Now that the bomb is placed you run to the escape pod to"
            print "get off this tin can."
            return 'escape_pod'

        else:
            print "DOES NOT COMPUTE!"
            return "the_bridge"

class EscapePod(Scene):

    def enter(self):
        print "You rush through the ship desperately trying to make it to"
        print "the escape pod before the whole ship explodes.  It seems like"
        print "hardly any Gothons are on the ship, so your run is clear of"
        print "interference.  You get to the chamber with the escape pods, and"
        print "now need to pick one to take.  Some of them could be damaged"
        print "but you don't have time to look.  There's 5 pods, which one"
        print "do you take?"

        good_pod = randint(1,5)
        guess = raw_input("[pod #]> ")


        if int(guess) != good_pod:
            print "You jump into pod %s and hit the eject button." % guess
            print "The pod escapes out into the void of space, then"
            print "implodes as the hull ruptures, crushing your body"
            print "into jam jelly."
            return 'death'

        else:
            print "You jump into pod %s and hit the eject button." % guess
            print "The pod easily slides out into space heading to"
            print "the planet below.  As it flies to the planet, you look"
            print "back and see your ship implode then explode like a"
            print "bright star, taking out the Gothon ship at the same"
            print "time.  You won!"

            return 'finished'

class Finished(Scene):

    def enter(self):
        print "You won! Good job."
        return 'finished'

class Map(object):

    scenes = {
        'central_corridor': CentralCorridor(),
        'laser_weapon_armory': LaserWeaponArmory(),
        'the_bridge': TheBridge(),
        'escape_pod': EscapePod(),
        'death': Death(),
        'finished': Finished(),
    }


    def __init__(self,  start_scene):
            self.start_scene = start_scene

    def next_scene(self, scene_name):
        val = Map.scenes.get(scene_name)
        return val

    def opening_scene(self):
        return self.next_scene(self.start_scene)

class Hero(object):
    def __init__(self, hero_energy):
        self.hero_energy = 3


class Weapons(object):
    weapons = {
        'feather': 'puppeteer',
        'tasp': 'kzin',
        'light_sabre': 'trooper',
        'grenade': 'predator',
    }

an_alien = Alien('puppeteer', 'central_corridor')
a_map = Map('central_corridor')
a_game = Engine(a_map)
a_game.play() 
Quote
#7
I like to view threads as specific to a specific issue, so that if someone else in the world has the same issue, it's easy for them to find the answer. A single thread for a single script doesn't make much sense.

That said, your Alien.load_alien only has one parameter... it should have two (self, and alien_lair). Then, instead of alien = Alien.aliens.get(alien_lair), it'd be alien = self.aliens[alien_lair].

Your scenes probably shouldn't be classes, or should maybe just be a single class that you instantiate multiple times, but that's unrelated :P
Quote
#8
I'll remember that the next time nilamo. Thanks.

So, I tried your suggestion, but I still got the same error:

Error:
Traceback (most recent call last):   File "ex43_mine.py", line 253, in <module>     a_game.play()   File "ex43_mine.py", line 44, in play     next_scene_name = current_scene.enter()   File "ex43_mine.py", line 66, in enter     loaded_alien = load_alien('central_corridor') NameError: global name 'load_alien' is not defined
My Aliens class now looks like this, with the rest the same as before:


class Alien(object):

    def __init__(self, alien_name, alien_lair):
        self.alien_name = alien_name
        self.alien_lair = alien_lair
        self.health = 2

        aliens = {
            'central_corridor': 'puppeteer',
            'laser_weapon_armory': 'kzin',
            'the_bridge': 'trooper',
            'escape_pod': 'predator',
        }

    # loads corresponding alien onto scene
    def load_alien(self, alien_lair):
        alien = self.aliens(alien_lair)
        return alien

Also, shouldn't get() be used to return the key value from the dict, which in this case is the alien itself? I'm using lines 230-232 as a point of reference here.

Cheers,
 
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#9
Your error in in line 66 (which is not shown), but you need to call the method load_alien in one of two (possibly more) ways
1. If the method that is calling load_alien is in the same class, call with self.load_alien(lair)
2. if called from another class, you need to call as <class>.load_alien(lair)
this will require creating an instance to the class you are calline, for example
[code]
my_class_name = Alien()
# then to call method
my_class_name.load_alien(lair)
nilamo likes this post
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#10
Thanks Larz60+.

Following your advice, I got it working using the following 2 lines:

an_alien = Alien('puppeteer', 'central_corridor')
loaded_alien = an_alien.load_alien('central_corridor')
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