How to make class immutable in python?

Question

I have read a lot about this subject here but i still can't find an appropriate answer. I have a class like:

class A(object):

    def __init__(self, first, second):
        self.first = first
        self.second = second

    def __eq__(self, other):
        return ****

    def __str__(self):
        return *****

    def __repr__(self):
        return **** 

a = A("a", "b")

How can i forbid a.first = "c" for example ?

I think thats the answer you're looking for - http://stackoverflow.com/questions/4828080/how-to-make-an-immutable-object-in-python :) — olgierdh, Mar 20 '14 at 20:45
This was the first think i read and I still can't apply it to my case. That is why i am posting my code. But still thanks. — user253956, Mar 20 '14 at 20:48

chepner · Accepted Answer · 2014-03-20T21:07:16.720

5

You can override __setattr__ to either prevent any changes:

def __setattr__(self, name, value):
    raise AttributeError('''Can't set attribute "{0}"'''.format(name))

or prevent adding new attributes:

def __setattr__(self, name, value):
    if not hasattr(self, name):
        raise AttributeError('''Can't set attribute "{0}"'''.format(name))
    # Or whatever the base class is, if not object.
    # You can use super(), if appropriate.
    object.__setattr__(self, name, value)

You can also replace hasattr with a check against a list of allowed attributes:

if name not in list_of_allowed_attributes_to_change:
    raise AttributeError('''Can't set attribute "{0}"'''.format(name))

Another approach is to use properties instead of plain attributes:

class A(object):

    def __init__(self, first, second):
        self._first = first
        self._second = second

    @property
    def first(self):
        return self._first

    @property
    def second(self):
        return self._second

edited Mar 20 '14 at 21:07

answered Mar 20 '14 at 21:01

chepner

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Okay, how can i raise an " AttributeError: can't set attribute " for example with __setattr__ – user253956 Mar 20 '14 at 21:04
Edited to raise an exception instead of doing nothing. – chepner Mar 20 '14 at 21:07
1

@user253956 unfortunately overriding `__setattr__` prevents you from setting the attributes even in `__init__`. You need something more sophisticated. – Mark Ransom Mar 20 '14 at 21:28
Yes, i faced that problem earlier. @property on the other hand works just fine. – user253956 Mar 20 '14 at 21:30

Mark Ransom · Answer 2 · 2014-04-23T14:20:26.870

You can disable __setattr__ as the last step of initializing the object.

class A(object):

    def __init__(self, first, second):
        self.first = first
        self.second = second
        self.frozen = True

    def __setattr__(self, name, value):
        if getattr(self, 'frozen', False):
            raise AttributeError('Attempt to modify immutable object')
        super(A, self).__setattr__(name, value)

>>> a = A(1, 2)
>>> a.first, a.second
(1, 2)
>>> a.first = 3

Traceback (most recent call last):
  File "<pyshell#46>", line 1, in <module>
    a.first = 3
  File "<pyshell#41>", line 10, in __setattr__
    raise AttributeError('Attempt to modify immutable object')
AttributeError: Attempt to modify immutable object

Edit: This answer has a flaw, which I'm sure is shared by every other solution: if the members themselves are mutable, nothing protects them. If your object contains a list for example, it's all over. This is in contrast to i.e. C++ where declaring an object const extends to all its members recursively.

How to make class immutable in python?

2 Answers2

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