Is there a reason to prefer list or tuple for __slots__?

Question

You can define __slots__ in new-style python classes using either list or tuple (or perhaps any iterable?). The type persists after instances are created.

Given that tuples are always a little more efficient than lists and are immutable, is there any reason why you would not want to use a tuple for __slots__?

>>> class foo(object):
...   __slots__ = ('a',)
... 
>>> class foo2(object):
...   __slots__ = ['a']
... 
>>> foo().__slots__
('a',)
>>> foo2().__slots__
['a']

Answer 1

First, tuples aren't any more efficient than lists; they both support the exact same fast iteration mechanism from C API code, and use the same code for both indexing and iterating from Python.

More importantly, the __slots__ mechanism doesn't actually use the __slots__ member except during construction. This may not be that clearly explained by the documentation, but if you read all of the bullet points carefully enough the information is there.

And really, it has to be true. Otherwise, this wouldn't work:

class Foo(object):
    __slots__ = (x for x in ['a', 'b', 'c'] if x != 'b')

… and, worse, this would:

slots = ['a', 'b', 'c']
class Foo(object):
    __slots__ = slots
foo = Foo()
slots.append('d')
foo.d = 4

For further proof:

>>> a = ['a', 'b']
>>> class Foo(object):
...     __slots__ = a
>>> del Foo.__slots__
>>> foo = Foo()
>>> foo.d = 3
AttributeError: 'Foo' object has no attribute 'd'
>>> foo.__dict__
AttributeError: 'Foo' object has no attribute '__dict__'
>>> foo.__slots__
AttributeError: 'Foo' object has no attribute '__slots__'

So, that __slots__ member in Foo is really only there for documentation and introspection purposes. Which means there is no performance issue, or behavior issue, just a stylistic one.