如何使用类方法获取类列表的实例变量？

Question

sorry if it is a too easy question but I am just a beginner in python and watching some tutorial videos on classes. I tried to add something by myself but couldn't. Here is the code :

class Dog:
    dogs = []

    def __init__(self, name):
        self.name = name
        self.dogs.append(self)
    
    @classmethod
    def num_dogs(cls):
        return len(cls.dogs)

    @classmethod
    def dogs_names(cls):
        for i in cls.dogs:
            return cls.dogs[i.name]
    
    @staticmethod
    def bark(n):
        """barks n times"""
        for _ in range(n):
            print("Bark! ")
 
        
tim = Dog("Tim")
jim = Dog("Jim")
print(Dog.num_dogs())
print(Dog.dogs_names())

So I know dogs_names() method looks dumb, but I hope you understand my point. I can reach the dogs list by only returning cls.dogs each time but how do I reach name variables of each item in the list?

Thank you for your help.

Answer 1

You can do :

class Dog:
    dogs = []

    def __init__(self, name):
        self.name = name
        self.dogs.append(self)
        
    @classmethod
    def num_dogs(cls):
        return len(cls.dogs)

    @classmethod
    def dogs_names(cls):
        names = []  # create a new list
        for i in range(len(cls.dogs)):
            names.append(cls.dogs[i].name)  # append each name one by one to the list
        return names  # return the full list at the end
        
    @staticmethod
    def bark(n):
        """barks n times"""
        for _ in range(n):
            print("Bark! ")
 
        
tim = Dog("Tim")
jim = Dog("Jim")
print(Dog.num_dogs())
print(Dog.dogs_names())

you can also use list conprehension :

    @classmethod
    def dogs_names(cls):
        return [dog.name for dog in cls.dogs]

Answer 2

There are two things wrong:

1. You are returning immediately, rather than allowing a list of results to be built.
2. i is an instance of Dog , so i.name is a dog name, not an index in to the list of Dog indices.

You build the list first, then return it after the loop is done building the list.

@classmethod
def dogs_names(cls):
    dog_names = []
    for i in cls.dogs:
        dog_names.append(i.name)
    return dog_names