CODEWITHSUNDEEP
python
user3543473
user3543473

Reputation: 41

What' the meaning of the brackets in the class?

In python, when I read others' code, I meet this situation where a class is defined and after it there is a pair of brackets.

class AStarFoodSearchAgent(SearchAgent):
     def __init__():
        #....

I don't know what is the meaning of '(SearchAgent)',because what I usually meet and use doesn't seem that.

Upvotes: 0

Views: 2317

Answers (5)

bigblind
bigblind

Reputation: 12867

It indicates that AStarFoodSearchAgent is a subclass of SearchAgent. It's part of a concept called inheritance.

What is inheritance?

Here's an example. You might have a Car class, and a RaceCar class. When implementing the RaceCar class, you may find that it has a lot of behavior that is very similar, or exactly the same, as a Car. In that case, you'd make RaceCar a subclass ofCar`.

class Car(object):
    #Car is a subclass of Python's base objeect. The reasons for this, and the reasons why you 
    #see some classes without (object) or any other class between brackets is beyond the scope 
    #of this answer.

    def get_number_of_wheels(self):
        return 4

    def get_engine(self):
        return CarEngine(fuel=30)

class RaceCar(Car):
#Racecar is a subclass of Car
    def get_engine(self):
        return RaceCarEngine(fuel=50)

my_car = Car() #create a new Car instance
desired_car = RaceCar() #create a new RaceCar instance.
my_car.get_engine() #returns a CarEngine instance
desired_car.get_engine() #returns a RaceCarEngine instance

my_car.get_number_of_wheels() #returns 4.
desired_car.get_number_of_wheels() # also returns 4! WHAT?!?!?!

We didn't define get_number_of_wheels on RaceCar, and still, it exists, and returns 4 when called. That's because RaceCar has inherited get_number_of_wheels from Car. Inheritance is a very nice way to reuse functionality from other classes, and override or add only the functionality that needs to be different.

Your Example

In your example, AStarFoodSearchAgent is a subclass of SearchAgent. This means that it inherits some functionality from SearchAgemt. For instance, SearchAgent might implement a method called get_neighbouring_locations(), that returns all the locations reachable from the agent's current location. It's not necessary to reimplement this, just to make an A* agent.

What's also nice about this, is that you can use this when you expect a certain type of object, but you don't care about the implementation. For instance, a find_food function may expect a SearchAgent object, but it wouldn't care about how it searches. You might have an AStarFoodSearchAgent and a DijkstraFoodSearchAgent. As long as both of them inherit from SearchAgent, find_food can use ìsinstanceto check that the searcher it expects behaves like aSearchAgent. Thefind_food`function might look like this:

def find_food(searcher):
    if not isinstance(searcher, SearchAgent):
        raise ValueError("searcher must be a SearchAgent instance.")

    food = searcher.find_food()
    if not food:
        raise Exception("No, food. We'll starve!")
    if food.type == "sprouts":
        raise Exception("Sprouts, Yuk!)
    return food

Old/Classic Style Classes

Upto Python 2.1, old-style classes were the only type that existed. Unless they were a subclass of some other class, they wouldn't have any parenthesis after the class name.

class OldStyleCar:
    ...

New style classes always inherit from something. If you don't want to inherit from any other class, you inherit from object.

class NewStyleCar(object):
    ...

New style classes unify python types and classes. For instance, the type of 1, which you can obtain by calling type(1) is int, but the type of OldStyleClass() is instance, with new style classes, type(NewStyleCar) is Car.

Upvotes: 3

falsetru
falsetru

Reputation: 368944

It means that SearchAgent is a base class of AStarFoodSearchAgent. In other word, AStarFoodSearchAgent inherits from SearchAgent class.

See Inheritance - Python tutorial.

Upvotes: 0

Alexander Starostin
Alexander Starostin

Reputation: 824

This is inheritance in python, just like in any other OO language

https://docs.python.org/2/tutorial/classes.html#inheritance

Upvotes: 0

Rami
Rami

Reputation: 7290

It means that class AStarFoodSearchAgent extends SearchAgent.

Check section 9.5 here

https://docs.python.org/2/tutorial/classes.html

Upvotes: 0

Epiglottal Axolotl
Epiglottal Axolotl

Reputation: 1048

SearchAgent is the superclass of the class AStarFoodSearchAgent. This basically means that an AStarFoodSearchAgent is a special kind of SearchAgent.

Upvotes: 0

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