Python - Property setting from list causes maximum recursion depth exceeded

Question

I have the following class:

class vehicle(object):
    def __init__(self, name):
        self.name = name
        self.kinds_list = ["tank", "car", "motorbike", "bike", "quad" ] 

    @property
    def kind(self):
        return self.kind

    @kind.setter
    def kind(self, x):
        if x in self.kinds_list:
            self.kind = x
        else:
            raise AttributeError('No attribute {0} found !'.format(y))

Setting kind causes the maximum recursion depth exceeded aka stack overflow.

Q: How to re-write the setter to make it work with fixed list only?

You want only certain allowed attributes? Maybe you should look at __slots__ instead... Either that or __setattr__... perhaps if you explained in a paragraph what you want to achieve... — Jon Clements♦
You do realise using self.kind = x in the setter calls the setter again? Use a different name for the underlying attribute and the property. — millimoose
@JonClements Not even remotely the cause of the stack overflow though. Your suggestion is useful in this specific case, but won't help the OP use @property correctly in general. — millimoose
Your getter and setter are recursively defined (self.kind = x calls the setter again, etc). Use a "private" attribute instead, e.g. self._kind. — Henry Keiter
Also, is there a reason you're using a checked kinds_list instead of just defining Tank, Car, [etc] classes that inherit from vehicle? — Henry Keiter

Bas Swinckels Bas Swinckels · Accepted Answer · 2013-09-24T18:35:01

The reason you reached the maximum recursion depth is that inside your setter, you do self.kind = ..., which recursively calls the same setter. You should store the value as some private attribute, just rename self.kind to self._kind.

class vehicle(object):
    def __init__(self, name):
        self.name = name
        self.kinds_list = ["tank", "car", "motorbike", "bike", "quad" ] 

    @property
    def kind(self):
        return self._kind

    @kind.setter
    def kind(self, x):
        if x in self.kinds_list:
            self._kind = x
        else:
            raise ValueError('{0} is an illegal kind of vehicle!'.format(y))

This is not a real private attribute like in other languages, since nothing prevents you from accessing my_vehicle._kind. By convention in python, everything starting with an underscore is private and shouldn't normally be touched outside of the class. Or as they say: python is for consenting adults ;).

I also slightly modified the error message in the setter.

Python - Property setting from list causes maximum recursion depth exceeded

1 Answers