Python 3类无法从函数中打印

Question

So i'm trying to wrap my head around the __init__ and functions. I am following the book, "Learn Python 3 the hard way" and I'm struggling to work out why my code won't print from a function but prints just fine from within the class.

from the code below:

class FirstScene(object):
    def enter(self):
        print("printing scene 1")

class SecondScene(object):
    pass

class Map(object):

    scenes = {
        "scene_one": FirstScene(),
        "scene_two": SecondScene()
    }

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

game_start = Map("scene_one")

When I run this. I don't get any output, but when I print directly from the class like this:

class FirstScene(object):
    print("printing scene 1")

It works. Any suggestions?

Answer 1

You never call the enter() method of FirstScene object, this is why nothing is printed.

Change self.scene = scene by self.scene = Map.scenes[scene] to get the actual object, then call its enter method (but be warned that this method is not defined for SecondScene objects!).

Another possibility is to rename enter of FirstScene to __init__ , so the print will be made upon instanciation. In that case, you have to decide whether you want to instanciate your objects when defining Map.scenes as you're currently doing, or in the __init__ method of Map , eg, using self.scene = Map.scenes[scene]() . In that case, you have to change the way you defined your dictionary on Map :

scenes = {
    "scene_one": FirstScene,
    "scene_two": SecondScene,
}

A more elegant way to do it:

class FirstScene(object):
    def __init__(self):
        print("printing scene 1")

class SecondScene(object):
    pass

class Map(object):
    def __init__(self, scene):
        self.scene = scene()

game_start = Map(FirstScene)

ie, you pass to the __init__ method of Map the class that has to be instanciated upon initialisation.

Answer 2

__init__() is short for "initialize" and is the python way of dealing with something called Constructors in object oriented programming.

I think it's best to use examples. Let's say you're creating a program called dog simulator.

# Notice that animals are a CLASS of objects.
class Animal:
        # Before we do anything, we have to create an instance of an object.
        # If we want to do something to a file, we have to first create one, aka (initialize one)
        # __init__ means "initialize", the two underscores in pythong mean it's
        # a special function.
        def __init__(self):
                print("initializing a new animal...")

        # One thing that animals do is they eat. It's a "function" of animals.  
        def eat(self):
                print("mmmm... delicious")

# Notice that dogs are a CLASS of animals.
class Dog(Animal):
        # one function of a dog is we have to create a new instance of it
        def __init__(self):
                print("initializing a new dog...")
        # another function is that they bar.
        def bark(self):
                print("woof!")

# This is our main program. Notice it's checking if the name of
# this process is called "main".
if __name__ == "__main__":
        doggo = Dog() # Lets create a new dog, we'll name him doggo
        doggo.eat() # Since dogs are a "class" of animals, they can do everything animals do.
        doggo.bark() # They can also do dog specific stuff.

Answer 3

Here's a change to your code that prints:

class FirstScene(object):
    def enter(self):
        print("printing scene 1")

class SecondScene(object):
    pass

class Map(object):

    scenes = {
        "scene_one": FirstScene(),
        "scene_two": SecondScene()
    }

    def __init__(self, scene):
        self.scene = self.scenes[scene]   ##### 1. Access the scene instance
        self.scene.enter()                ##### 2. Tell it to print

game_start = Map("scene_one")

In Map.__init__ , scene is the string "scene_one" At #1 above, you need to actually use the scenes dictionary that you created to get the FirstScene() or SecondScene() instance.

Once you have an instance, at #2 above, you call the enter() method to cause whatever enter() does to happen. In this case, that is printing printing scene 1 .