Using Properties in Python classes cause "maximum recursion depth exceeded" [duplicate]

Question

Here is a test class that I wrote to became familiar with @properties and setter functionality in Python script:

class Test(object):
    def __init__(self, value):
        self.x =  value
    @property
    def x(self):
        return self.x
    @x.setter
    def x(self, value):
        self.x = value

The problems is that when I want to create an object from my class, I face the following error:

>>> t = Test(1)

Traceback (most recent call last):
  File "<pyshell#19>", line 1, in <module>
    t = Test(1)
  File "<pyshell#18>", line 3, in __init__
    self.x =  value
  File "<pyshell#18>", line 9, in x
    self.x = value
  File "<pyshell#18>", line 9, in x
  #A bunch of lines skipped
RuntimeError: maximum recursion depth exceeded
>>>

Use self._x instead of self.x for private member. By naming both member and property x property shadows member, and return self.x in getter body calls itself. — Łukasz Rogalski
– Łukasz Rogalski, Commented Apr 29, 2016 at 6:53

Reblochon Masque · Accepted Answer · 2016-04-29 06:55:21Z

55

You are using the same name for the getter, setter and attribute. When setting up a property, you must rename the attribute locally; the convention is to prefix it with an underscore.

class Test(object):
    def __init__(self, value):
        self._x =  value

    @property
    def x(self):
        return self._x

answered Apr 29, 2016 at 6:55

Reblochon Masque

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5 Comments

EbraHim Over a year ago

Thank you. Please take a look at Byte Commander's answer here. He used same name for setter, getter and variable, but his program works fine.

Rob Over a year ago

If you've defined the setter method, it's better to only access the private variable self._x in the getter / setter / deleter methods. In __init__ you can just say self.x = value.

Open AI - Opting Out Over a year ago

@Rob I'm not sure that's correct. Why is it better? It's not a convention I'm aware of.

Rob Over a year ago

You are implementing a setter methode for a reason, probably to do some input checking / manipulation. It makes sense to do the same thing during instantiation of the object.

Alex Povel Over a year ago

Yes, exactly. Assigning to the private attribute in __init__ will bypass the property setter method and therefore not apply the logic contained there, as @Rob said. It is probably buggy or at least inconsistent to allow potentially different values during instantiation and later one, during re-assignment.

Reblochon Masque · Accepted Answer · 2019-07-13 11:42:38Z

-3

The problem is in these lines:

def x(self):
    return self.x

Replace it with

def get_x(self):
    return self.x

Because now the function calls itself, leading to the recursion depth exceeded.

edited Jul 13, 2019 at 11:42

Reblochon Masque

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answered Apr 29, 2016 at 6:52

Mathias711

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2 Comments

EbraHim Over a year ago

I want to use @properties.

Guzman Ojero Over a year ago

Using getters/setters is a Python antipattern. It's recommended to uses properties with the @property decorator. Read: pythonmorsels.com/…

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