How to dynamically change __slots__ attribute?

Question

Suppose I have a class with __slots__

class A:
    __slots__ = [\'x\']

a = A()
a.x = 1   # works fine
a.y = 1   # AttributeError (as expected)
         


        

Answer 1

Putting answers to this and related question together, I want to make an accent on a solution to this problem:

You can kind of modify __slots__ by creating a subclass with the same name and then replacing parent class with its child. Note that you can do this for classes declared and used in any module, not just yours!

---

Consider the following module which declares some classes:

module.py:

class A(object):
    # some class a user should import
    __slots__ = ('x', 'b')

    def __init__(self):
        self.b = B()

class B(object):
    # let's suppose we can't use it directly,
    # it's returned as a part of another class
    __slots__ = ('z',)

Here's how you can add attributes to these classes:

>>> import module
>>> from module import A
>>>
>>> # for classes imported into your module:
>>> A = type('A', (A,), {'__slots__': ('foo',)})
>>> # for classes which will be instantiated by the `module` itself:
>>> module.B = type('B', (module.B,), {'__slots__': ('bar',)})
>>>
>>> a = A()
>>> a.x = 1
>>> a.foo = 2
>>>
>>> b = a.b
>>> b.z = 3
>>> b.bar = 4
>>>

But what if you receive class instances from some third-party module using the module?

module_3rd_party.py:

from module import A

def get_instance():
    return A()

No problem, it will also work! The only difference is that you may need to patch them before you import third-party module (in case it imports classes from the module):

>>> import module
>>>
>>> module.A = type('A', (module.A,), {'__slots__': ('foo',)})
>>> module.B = type('B', (module.B,), {'__slots__': ('bar',)})
>>>
>>> # note that we import `module_3rd_party` AFTER we patch the `module`
>>> from module_3rd_party import get_instance
>>>
>>> a = get_instance()
>>> a.x = 1
>>> a.foo = 2
>>>
>>> b = a.b
>>> b.z = 3
>>> b.bar = 4
>>>

It works because Python imports modules only once and then shares them between all other modules, so the changes you make to modules affect all code running along yours.