Prevent creating new attributes for class or module

Question

I saw this question on SO Prevent creating new attributes outside init which shows how to prevent adding new attributes to objects of classes.

I wanted the same behaviour for the overall class or even the complete loaded module.

Example class:

class Klass:
     a = 0
     b = 1

Another module:

from Klass import Klass

Klass.c = 2 # this should raise an error

Is this possible?

score 5 · Accepted Answer · answered Dec 17 '19 at 14:45

5

If you're trying to prevent modifying the class itself, you can create a metaclass that defines the __setattr__ method for the class.

class FrozenMeta(type):
    def __new__(cls, name, bases, dct):
        inst = super().__new__(cls, name, bases, {"_FrozenMeta__frozen": False, **dct})
        inst.__frozen = True
        return inst
    def __setattr__(self, key, value):
        if self.__frozen and not hasattr(self, key):
            raise TypeError("I am frozen")
        super().__setattr__(key, value)

class A(metaclass=FrozenMeta):
    a = 1
    b = 2

A.a = 2
A.c = 1 # TypeError: I am frozen

answered Dec 17 '19 at 14:45

Patrick Haugh

59,226
13
88
96

This works as expected for the class! Too bad, there isn't something for the whole module – RoQuOTriX Dec 17 '19 at 14:52
@RoQuOTriX: Check out [this answer](https://stackoverflow.com/a/7668273/208880) for a way to make a class be a module. – Ethan Furman Dec 24 '19 at 22:40

Philippe Dixon · Answer 2 · 2019-12-17T14:46:57.087

-1

The answer with slots would be the Pythonic way to do it.

class Klass:
    __slots__ = ['a', 'b']

    def __init__(self, a=0, b=1):
        self.a = a
        self.b = b

>>> k = klass.Klass()
>>> k.a
0
>>> k.b
1
>>> k.c = 2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Klass' object has no attribute 'c'
>>>

edited Dec 17 '19 at 14:46

answered Dec 17 '19 at 14:40

Philippe Dixon

87
3

But you can still do Klass.c=2 and then k will also have c – Martin Dec 17 '19 at 14:49
1

`__slots__` were not designed to prevent attribute creation, that is merely a side-effect. – Ethan Furman Dec 24 '19 at 19:26

Prevent creating new attributes for class or module

2 Answers2

Linked