Automatic function call on instance variable of Python class?

Question

So lets say I have a class called Car as below:

class Car(object):
    """Car example with weight and speed."""
    def __init__(self):
        self.weight = None
        self.speed = None

If I initialize a Car as an empty object:

red_car = Car()

And I add a speed and a weight:

red_car.speed = 60
red_car.weight = 3500

That's all fine but what if I want to run a function when attempting to add those variables to the instance? Like this function:

def change_weight(self, any_int):
    return (any_int - 10)

The thing is though I want it to automatically run this function whenever I attempt to add specific instance variables to the object. If possible I would like it to run change_weight only on the weight instance variable.

Am I understanding this correctly or should I just be running the integer through the function separately and then adding to the object after manually?

This seems like a perfect use case for getters and setters.

GWW
– GWW

2014-05-09 01:34:43 +00:00
Commented May 9, 2014 at 1:34 — GWW
– GWW, Commented May 9, 2014 at 1:34
@GWW, Noooo. This is Python not Java

John La Rooy
– John La Rooy

2014-05-09 01:35:43 +00:00
Commented May 9, 2014 at 1:35 — John La Rooy
– John La Rooy, Commented May 9, 2014 at 1:35
@gnibbler: Sorry I should have said properties

GWW
– GWW

2014-05-09 03:50:36 +00:00
Commented May 9, 2014 at 3:50 — GWW
– GWW, Commented May 9, 2014 at 3:50

Antti Haapala · Accepted Answer · 2014-05-09 01:44:55Z

6

You want to use properties

class Car(object):
    """Car example with weight and speed."""
    def __init__(self):
        self._weight = None # internal value
        self.speed = None

    @property
    def weight(self):
        return self._weight

    @weight.setter
    def weight(self, the_weight):
        self._weight = the_weight - 10 # or whatever here

Now, you can set speed normally; when you execute car.weight = 20 the setter function will be called, and the actual weight will be set to 20 - 10 = 10.

answered May 9, 2014 at 1:44

Antti Haapala

135k23 gold badges297 silver badges349 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Matthew Over a year ago

All this time.. so wait a sec let me see if I understand this correctly then.. You're declaring the function called weight with a decorator property but it's returning to the instance _weight (why a private value? Why not just weight? Then you have another function called weight but this has a decorator called weight.setter on it, I assume because then when I do car.weight = 20 it runs this function? And the decorator weight.setter grabs the other weight function as a return once the weight.setter function is done? For the most part it makes sense.

Matthew Over a year ago

I just am looking for clarification that what I wrote above is correct and the whole thing about having to make it a private variable with the _.

dimo414 · Accepted Answer · 2014-05-09 02:09:51Z

2

Read up on Python's notion of Descriptors. In particular, it sounds like you want to define a custom __set__() method.

For example, obj.d looks up d in the dictionary of obj. If d defines the method__get__(), then d.__get__(obj) is invoked according to the precedence rules listed below.

Alternately, you may find using Properties (a specific type of Descriptor) a little easier to work with.

property() is a succinct way of building a data descriptor that triggers function calls upon access to an attribute.

The behavior of descriptors is fairly logical once you understand it, but it doesn't behave quite as you might expect; in particular, __get__() and __set__() should not be confused with getters and setters, like the Java concept. So look at some of the examples above and play around with them before diving in.

edited May 9, 2014 at 2:09

answered May 9, 2014 at 1:54

dimo414

49.3k19 gold badges167 silver badges273 bronze badges

1 Comment

Matthew Over a year ago

Interesting! Will do!

Collectives™ on Stack Overflow

Automatic function call on instance variable of Python class?

2 Answers 2

2 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related