Python call a list from outside the method and class then return what I expect-CodePudding

I want to print at the end of code, something like this: (car1, car2)

But the loop I created at the end, can't find the the new_list because it is inside the class Vehicles.

class Vehicles:
    color = ''
    wheels = 0
    
    def __init__(self, color, wheels):
        self.color = color
        self.wheels = wheels
    
    def filtering(self, list_vehicles, vehicle):
        new_list = []
        new_list = list(filter(lambda x: True if x.__class__.__name__ == vehicle else False, list_vehicles))
        return (new_list)
    
    def __str__(self):
        print(f"This bus is {self.color} and has {self.wheels} wheels.")
    
bus1 = Vehicles("white", 6)
bus1.__str__()

class Car(Vehicles):
    speed = 0
    changes = 0
    
    def __init__(self, color, wheels, speed, changes):
        super().__init__(color, wheels)
        self.speed = speed
        self.changes = changes
    
    def __str__(self):
        print(f"This car is {self.color}, has {self.wheels} wheels, his speed is {self.speed} mph and it has {self.changes} changes.")

car1 = Car("red", 4, 110, 5)
car1.__str__()
car2 = Car("blue", 4, 130, 6)
car2.__str__()

filtering([bus1, car1, car2], "Car")

for x in new_list:
    print(x)

CodePudding user response：

Basically you have to use a class object to call a class function or you have to use a static method instead.One more thing you are returning a list from your function and not saving it anywhere. You have to make a new variable named new_list to save that, so that you can traverse over it. So if you make this change, it will run fine.

new_list  = car1.filtering([bus1, car1, car2], "Car")

CodePudding user response：

You are using an instance method and yet you don't provide it with the access to the instance object.

Here would be one way of doing it.

class Vehicles:
    color = ''
    wheels = 0
    
    def __init__(self, color, wheels):
        self.color = color
        self.wheels = wheels
    
    def filtering(self, list_vehicles, vehicle):
        new_list = []
        new_list = list(filter(lambda x: True if x.__class__.__name__ == vehicle else False, list_vehicles))
        return new_list
    
    def __str__(self):
        return f"This bus is {self.color} and has {self.wheels} wheels."
    
bus1 = Vehicles("white", 6)
bus1.__str__()

class Car(Vehicles):
    speed = 0
    changes = 0
    
    def __init__(self, color, wheels, speed, changes):
        super().__init__(color, wheels)
        self.speed = speed
        self.changes = changes
    
    def __str__(self):
        return f"This car is {self.color}, has {self.wheels} wheels, his speed is {self.speed} mph and it has {self.changes} changes."

car1 = Car("red", 4, 110, 5)
car1.__str__()
car2 = Car("blue", 4, 130, 6)
car2.__str__()

new_list  = car1.filtering([bus1, car1, car2], "Car")

for x in new_list:
  print(x)

I have fixed some other issues with the code as well. One of them being, not returning anything from __str__ (dunder str method). You should return the string representation actually. As you didn't return anything, python just got the implicit return of None.

Edit:

That instance method is not even using anything instance specific. So you can simply make it a static method or class method. For brevity we show the example with class method. As mentioned by @chepner) this solution would be better than passing the string and using __class__ is to use the isinstance() like this. Notice that here we have made the method a class method. As a result, the first parameter denotes or object denoting the class itself.

class Vehicles:
    color = ''
    wheels = 0
    
    def __init__(self, color, wheels):
        self.color = color
        self.wheels = wheels
    @classmethod
    def filtering(cls,  list_vehicles):
        new_list = []
        new_list = list(filter(lambda x: True if isinstance(x,cls) else False, list_vehicles))
        return new_list
    
    def __str__(self):
        return f"This bus is {self.color} and has {self.wheels} wheels."
    
bus1 = Vehicles("white", 6)
bus1.__str__()

class Car(Vehicles):
    speed = 0
    changes = 0
    
    def __init__(self, color, wheels, speed, changes):
        super().__init__(color, wheels)
        self.speed = speed
        self.changes = changes
    
    def __str__(self):
        return f"This car is {self.color}, has {self.wheels} wheels, his speed is {self.speed} mph and it has {self.changes} changes."

car1 = Car("red", 4, 110, 5)
car1.__str__()
car2 = Car("blue", 4, 130, 6)
car2.__str__()

new_list  = car1.filtering([bus1, car1, car2])

for x in new_list:
  print(x,",")