如何使用字典式创建对象？

Question

I'm new in stackoverflow and I'd like to make my first question for a problem in this code I've tried to write to learn objects in python. I'm trying to call the creation of an object through a dictionary. My purpose is to create an object thanks to a number, for example I have the dictionary newch = {1 : Character.new_dragon(), 2 : Character.new_goblin()} and when I call Player1 = newch[1] it should create a new dragon (@classmethod new_dragon) and assign it to Player1

The problem is that when i run the program, Character.new_dragon() and Character.new_goblin() are called automatically (i put a control print), but when I write "DRAGO" after the request "which player?" the functions aren't called because there isn't the control print

import random

class Character:
    def __init__(self,idd,height,weight,att,defe):
        self.idd=idd
        self.height=height
        self.weight=weight
        self.att=att
        self.defe=defe


    @classmethod
    def new_goblin(cls):
        print('newgoblin')
        return cls(1,getr(1,1.5,0.1),getr(40,60,0.5),getr(5,15,1),getr(6,10,1))

    @classmethod
    def new_dragon(cls):
        print('newdrago')
        return cls(2,getr(20,30,1),getr(500,2000,5),getr(50,150,3),getr(20,100,3))


def getr(start,stop,step):          #returns float
    x=random.randint(1, 1000)
    random.seed(x)
    return random.randint(0, int((stop - start) / step)) * step + start


play={1:'p1', 2:'p2', 3:'p3', 4:'p4'} #dict for players
newch={1:Character.new_dragon(),2:Character.new_goblin()} ############This doesn't work


i=1       
while True:
    char=input("which player? Drago or Goblin?").upper()
    if(char=="DRAGO"):
        play[i]=newch[1]  #here i try to call Character.new_dragon()
        i+=1
        break
    elif(char=="GOBLIN"):
        play[i]=newch[2]
        i+=1
        break
    print("write \'Drago\' or \'Goblin\'")

print(play[1].height, play[1].weight, play[1].att, play[1].defe)

Here's my code, if you could help me, I would be very glad, thanks

Answer 1

The new object is created immediately when you call Character.new_dragon() , and the object is then stored in the dict.

Instead you could not store the object in the dict, but the function that creates it. That function would be Character.new_dragon (without the () ). Then you can call that function when the player selects a character: play[i]=newch[1]()

Complete code:

import random

class Character:
    def __init__(self,idd,height,weight,att,defe):
        self.idd=idd
        self.height=height
        self.weight=weight
        self.att=att
        self.defe=defe


    @classmethod
    def new_goblin(cls):
        print('newgoblin')
        return cls(1,getr(1,1.5,0.1),getr(40,60,0.5),getr(5,15,1),getr(6,10,1))

    @classmethod
    def new_dragon(cls):
        print('newdrago')
        return cls(2,getr(20,30,1),getr(500,2000,5),getr(50,150,3),getr(20,100,3))


def getr(start,stop,step):          #returns float
    x=random.randint(1, 1000)
    random.seed(x)
    return random.randint(0, int((stop - start) / step)) * step + start


play={1:'p1', 2:'p2', 3:'p3', 4:'p4'} #dict for players
newch={1:Character.new_dragon,2:Character.new_goblin} ############This doesn't work


i=1       
while True:
    char=input("which player? Drago or Goblin?").upper()
    if(char=="DRAGO"):
        play[i]=newch[1]()  #here i try to call Character.new_dragon()
        i+=1
        break
    elif(char=="GOBLIN"):
        play[i]=newch[2]()
        i+=1
        break
    print("write \'Drago\' or \'Goblin\'")

print(play[1].height, play[1].weight, play[1].att, play[1].defe)

This works, however I would not say it is the best coding style. Its hard to judge from only this piece of code, but it might be a better idea to make Drago and Goblin subclasses of the Character class and store the type of those classes in that dictionary.

Answer 2

When you are initialising the dictionary this way:

newch={1:Character.new_dragon(),2:Character.new_goblin()}

You are binding keys (1 and 2) to the return values of the new_dragon and new_goblin functions. You need to bind the functions(without calling them) like so:

newch={1:Character.new_dragon,2:Character.new_goblin} Notice there are no brackets!

And then, when you create players, you execute those functions like so:

play[i]=newch[1]() Notice here we have brackets!

Additionally, if I may suggest an improvement of the code here:

if(char=="DRAGO"):
    play[i]=newch[1]()
    i+=1

To avoid the if statement, you can create you mapping with a string:

newch={"DRAGO":Character.new_dragon,"GOBLIN":Character.new_goblin}

And create instances just by calling

play[i]=newch[char]()

To handle errors, you can add just a single if statement checking whether the char string is in the list with dict keys.

Answer 3

newch={1:Character.new_dragon(),2:Character.new_goblin()}

As this is written, the new_dragon and new_goblin functions are called when the dictionary is created. This is why you are seeing them both run "automatically" every time you run your program.

If you instead declared the dict like:

newch={1:Character.new_dragon ,2:Character.new_goblin}

And later have something like:

if(char=="DRAGO"):
    play[i]=newch[1]()

(note the parenthesis after the newch[1] ) you should get what you want.

Incidentally, those break statements aren't necessary. The If/elif/else chain doesn't fall through like a switch statement in other languages.