Using Properties in Python classes cause "maximum recursion depth exceeded"

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, Apr 29 '16 at 06:53

score 50 · Accepted Answer · answered Apr 29 '16 at 06:55

50

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 '16 at 06:55

Reblochon Masque

35,405
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Thank you. Please take a look at _Byte Commander_'s answer [here](http://stackoverflow.com/questions/36929460/how-to-keep-variable-and-dictionary-that-contains-it-consistent/36930474?noredirect=1#comment61422678_36930474). He used same name for `setter`, `getter` and `variable`, but his program works fine. – EbraHim Apr 29 '16 at 07:02
2

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`. – Rob Apr 29 '16 at 07:02
@Rob I'm not sure that's correct. Why is it better? It's not a convention I'm aware of. – Peter Wood Apr 29 '16 at 07:12
2

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. – Rob Apr 29 '16 at 07:16
1

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. – Alex Povel Apr 29 '20 at 14:42

score -3 · Answer 2 · edited Jul 13 '19 at 11:42

-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 '19 at 11:42

Reblochon Masque

35,405
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answered Apr 29 '16 at 06:52

Mathias711

6,568
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I want to use `@properties`. – EbraHim Apr 29 '16 at 06:55
1

Using getters/setters is a Python antipattern. It's recommended to uses properties with the @property decorator. Read: https://www.pythonmorsels.com/customizing-what-happens-when-you-assign-attribute/ – Guzman Ojero May 16 '22 at 22:48

Using Properties in Python classes cause "maximum recursion depth exceeded"

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