How to protect class attributes in Python?

Question

How to protect class from adding attributes in that way:

class foo(object):
     pass

x=foo()
x.someRandomAttr=3.14

Do you want an object without any attributes? Then just use x = object(). — Martijn Pieters
– Martijn Pieters, Commented Oct 26, 2013 at 14:14
@MartijnPieters - No, I think he is making a class and wants it so that nobody can add anything to it. In other words, you get only what comes with the class (or namely what he put in it). — user2555451
– user2555451, Commented Oct 26, 2013 at 14:16
@iCodez: That's why I made that a comment; the question is rather sparse on details. — Martijn Pieters
– Martijn Pieters, Commented Oct 26, 2013 at 14:19

Martijn Pieters · Accepted Answer · 2013-10-26 14:34:22Z

4

If you want an immutable object, use the collections.namedtuple() factory to create a class for you:

from collections import namedtuple

foo = namedtuple('foo', ('bar', 'baz'))

Demo:

>>> from collections import namedtuple
>>> foo = namedtuple('foo', ('bar', 'baz'))
>>> f = foo(42, 38)
>>> f.someattribute = 42
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'foo' object has no attribute 'someattribute'
>>> f.bar
42

Note that the whole object is immutable; you cannot change f.bar after the fact either:

>>> f.bar = 43
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: can't set attribute

edited Oct 26, 2013 at 14:34

answered Oct 26, 2013 at 14:14

Martijn Pieters

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Comments

Ashwini Chaudhary · Accepted Answer · 2013-10-26 14:13:02Z

3

Override the __setattr__ method:

>>> class Foo(object):
    def __setattr__(self, var, val):
        raise TypeError("You're not allowed to do this")
...     
>>> Foo().x = 1
Traceback (most recent call last):
  File "<ipython-input-31-be77d2b3299a>", line 1, in <module>
    Foo().x = 1
  File "<ipython-input-30-cb58a6713335>", line 3, in __setattr__
    raise TypeError("You're not allowed to do this")
TypeError: You're not allowed to do this

Even Foo's subclasses will raise the same error:

>>> class Bar(Foo):
    pass
... 
>>> Bar().x = 1
Traceback (most recent call last):
  File "<ipython-input-35-35cd058c173b>", line 1, in <module>
    Bar().x = 1
  File "<ipython-input-30-cb58a6713335>", line 3, in __setattr__
    raise TypeError("You're not allowed to do this")
TypeError: You're not allowed to do this

answered Oct 26, 2013 at 14:13

Ashwini Chaudhary

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3 Comments

chepner Over a year ago

This prevents you from setting any attributes, including in __init__().

tdelaney Over a year ago

@chepner -you can still set attrs through the ____dict___. Ugly, but doable.

martineau Over a year ago

@chepner: What @tdelaney meant was you simply need to use self.__dict__['<attribute-name>'] = <whatever>. It's a well-know technique used in classes that redefine __setattr__.

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