Is it possible to "dynamically" modify an instance attribute? Possibly by matching a strin-CodePudding

Essentially I'd like to write a setter that can modify different class variables depending on input. For example:

@dataclass
class Rocket:
    fuel: int
    oxidizer: int
    weight: int
    power: int
    # I actually have tons of variables

    def decrease_value(self, value, amount):
        # magically match value to self.`value`
        # subtract amount from the matching class variable

r = Rocket
r.decrease_value('fuel', 200)
r.decrease_value('weight', 55)

Something like this would be much nicer than the huge else if chain that I'd rather not write.

CodePudding user response：

Yes, you can, but note that this is not the best practice as static code analyzers won't be able to find your bugs:

from dataclasses import dataclass

@dataclass
class Rocket:
    fuel: int
    oxidizer: int
    weight: int
    power: int
    # I actually have tons of variables

    def decrease_value(self, value, amount):
        if value in self.__dict__:    # Check if the attribute name is valid
            self.__dict__[value] -= amount


r = Rocket(fuel=1000, oxidizer=2000, weight=3000, power=4000)
r.decrease_value('fuel', 200)
r.decrease_value('weight', 55)
r.decrease_value('foobar', 55)   # This will not cause an error

print(r)

This will print

Rocket(fuel=800, oxidizer=2000, weight=2945, power=4000)

CodePudding user response：

You can achieve this using through getattr and setattr methods like

class Rocket: 
    fuel: int = 0 
    oxidizer: int = 0 
    weight: int = 0 
    power: int = 0 
    # I actually have tons of variables 
 
    def __init__(self, fuel, oxidizer, weight, power):
        # initialize the initial value
        self.fuel = fuel 
        self.oxidizer = oxidizer 
        self.weight = weight 
        self.power = power 

    def decrease_value(self, value, amount): 
        # magically match value to self.`value` 
        # subtract amount from the matching class variable 
        try: 
            value_is = getattr(self, value) 
            setattr(self, value, (value_is - amount)) 
        except AttributeError: 
            print(f'{value} variable is not available in Rocket class') 
 
r = Rocket(fuel=100, oxidizer=100, weight=100, power=100) 
r.decrease_value('fuel', 200) 
r.decrease_value('weight', 55)

see the getattr and setattr method in detail it will give you more understanding