If I have an object that is a property of another object, how can I reference the main object from the property object?

Question

Apologies if title was a little unclear. A little shaky on Python object-oriented fundamentals, but here is a basic example: Let's say I have a "Body" class and a "Legs" class, and I have an instance of "Legs" (rightLeg) as a property of a "Body" instance (georgesBody). If I call a function like rightLeg.break(), how can I, inside of break() for the "Legs" class, ping the corresponding "Body" instance and pass it some information (maybe through a setter method of "Body" set up to accept said ping)?

I need to be able to trigger code that calls the specific instance of "Body", georgesBody, from within rightLeg dynamically (without referencing the actual name georgesBody). Any help is much appreciated!

score 3 · Accepted Answer · answered Nov 23 '20 at 20:08

3

I would define the legs inside the __init__ method of Body, then link them to eachother by requiring a body argument for the legs.

class Body:
    def __init__(self):
        self.left_leg = _Leg(self)
        self.right_leg = _Leg(self)

class _Leg:
    def __init__(self, body):
        self.body = body

    def break_leg(self):
        print(self, self.body)

Body().left_leg.break_leg()

answered Nov 23 '20 at 20:08

Mandera

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Thanks a ton!! Does the underscore before "Leg" in the class declaration mean anything syntax-wise? – quantumchocolate Nov 23 '20 at 21:20
1

Glad I could help! No it's just a [naming convention](https://stackoverflow.com/a/1301369/3936044) for a weak "internal use" indicator – Mandera Nov 24 '20 at 06:19

score 1 · Answer 2 · answered Nov 23 '20 at 20:11

What you want to do could be done by a publish/subscribe mechanism with messages

Simpler is to provide the body upon instantiating the legs - but that way you also have tighter coupling and legs need to know about the bodys methods it can call:

class Leg: 
    def __init__(self, side, body): 
        self.side = side
        self.body = body

    def destroy(self):
        # call something on the body
        self.body.shout("Leg broken: " + self.side)

class Body: 
    def __init__(self): 
        self.legs=[Leg("left", self), Leg("right", self)]
    def shout(self, msg):
        print(msg)


b = Body()
b.legs[0].destroy()
b.legs[1].destroy()

Output:

Leg broken: left
Leg broken: right

Thank you for the detailed example, passing the body itself to the leg was not something I thought of. Thanks! — quantumchocolate, Nov 23 '20 at 21:25

If I have an object that is a property of another object, how can I reference the main object from the property object?

2 Answers2