Unchangeable variables in a class?

[Calab]

I want a class that will accept a value, and provide some other values based on calculations from the original value. I do not want these calculated values to be changeable.

Sample code (Python 2.7):

class MyNum:
    def __init__(self, value):
        self.value = value
        self.double = self.value * 2

x = MyNum(2)
print(x.value, x.double)
x.double = 8
print(x.value, x.double)

Output is:
(2, 4)
(2, 8)

I would like to see an error when I try to assign x.double a value. How can I prevent my class variable from being changed?

[Calab]

From what I find online, the following code SHOULD do what I want, but it still isn't working as expected:

class MyNum:
    
    def __init__(self, value):
        self._value = value
        self._double = value * 2
        
    @property
    def value(self):
        return self._value
    
    @property
    def double(self):
        return self._double
    
x = MyNum(2)
print(x.value, x.double)
x.double = 8
print(x.value, x.double)

The output is still:
(2, 4)
(2, 8)

[Gribouillis]

You must be running some other code. My output for your code is

Error:
λ python paillasse/pf/calab.py
2 4
Traceback (most recent call last):
  File "/home/eric/Projets/Scratch/2023-01/paillasse/pf/calab.py", line 17, in <module>
    x.double = 8
AttributeError: can't set attribute 'double'

as it should be.

I hope you are not running Python 2, are you?

[menator01]

Still changes

class MyNum:
     
    def __init__(self, value):
        self._value = value
        self._double = value * 2
         
    @property
    def value(self):
        return self._value
     
    @property
    def double(self):
        return self._double
     
x = MyNum(2)
print(x.value, x.double)
x._double = 8
print(x.value, x.double)

Output:
2 4
2 8

[Gribouillis]

Still changes

How are you using Python, which version, which setup? There must be some mistake.

[menator01]

Version 3.10 on ubuntu with vscode editor
I've been doing a little reading on this, from what I understand that there is no way to prevent either class or instance variables from being changed.
The closest thing I found was some thing about using __slots__

[Calab]

You must be running some other code. My output for your code is

Error:
λ python paillasse/pf/calab.py
2 4
Traceback (most recent call last):
  File "/home/eric/Projets/Scratch/2023-01/paillasse/pf/calab.py", line 17, in <module>
    x.double = 8
AttributeError: can't set attribute 'double'

as it should be.

I hope you are not running Python 2, are you?

I am running Python 2.7.13, only because the system we are working on needs to support a number of old python scripts.

Your output is what I would expect to get, but that's not occurring here. I am running this in a Jupyter notebook, but that shouldn't impact the behaviour here.

[Gribouillis]

Version 3.10 on ubuntu

I can't believe it. Can you add

import sys
print(sys.version)

[menator01]

import sys
print(sys.version)

class MyNum:
      
    def __init__(self, value):
        self._value = value
        self._double = value * 2
          
    @property
    def value(self):
        return self._value
      
    @property
    def double(self):
        return self._double
      
x = MyNum(2)
print(x.value, x.double)
x._double = 8
print(x.value, x.double)

Output:
3.10.12 (main, Jun 11 2023, 05:26:28) [GCC 11.4.0]
2 4
2 8

[Gribouillis]

I am running Python 2.7.13

You shouldn't run Python 2.7. However a solution to your problem is to write

class MyNum(object):
    ...

In Python 2, classes that don't subclass object explicitly are "old style classes", they actually belong to Python 1 and don't behave the same way. We're back more than 20 years ago with your code.

@menator01 The problem is that you're using x._double = 8 instead of x.double = 8. You are breaking the encapsulation.