`__slots__ = ()` missing in many base classes?

[ariebovenberg]

related: #416

It seems an empty __slots__ is forgotten in most base classes (like Immutable, Equable, etc.), greatly reducing the advantage (i.e. memory savings) of defining __slots__ in their subclasses (e.g. Result, Maybe). Unless I'm missing something, this is not intended. In that case I will gladly submit a PR fixing this.

Example

>>> from returns.result import Success  # Result defines __slots__
>>> a = Success(4)
>>> a.__dict__  # this should not be present, but it is inherited anyway from base classes without __slots__
{}
>>> object.__setattr__(a, 'foo', 9)  # We can technically set any attribute because there is a `__dict__`
>>> a.foo
9
>>> a.__dict__
{'foo': 9}

Expected behavior of slots classes

>>> class Base:
...  __slots__ = ()  # necessary!
>>> class A(Base):
...  __slots__ = ('foo', )
...
>>> a = A()
>>> a.foo = 4
>>> a.bar = 9
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'A' object has no attribute 'bar'
>>> a.__dict__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'A' object has no attribute '__dict__'

[sobolevn]

Yes, my bad. I forgot that I need empty __slots__. PRs are welcome! 👍

[ariebovenberg]

@sobolevn it seems the is_error_handled helper of the pytest plugin patches success/failure containers with an extra _error_handled attribute which is not used outside of tests.

My suggestion would be to add this property to __slots__. How do you feel about adding test-specific behavior to non-test code?

The other alternatives I can think of to avoid this extra __slots__ field are even worse:

• In the pytest plugin, do a full patch of the classes, substituting them completely with ones with more slots. Not sure if this would even work, but even so it'd be hacky as hell.
• set __slots__ to include the extra property only if pytest module can be found. Also quite hacky.
• abandon slots for most containers
• abandon this pytest plugin helper

[ariebovenberg]

🤔 One other option would be to create a separate (<object id>, <object>): <is error handled> mapping in the test plugin. That'd avoid the extra attribute but we'd have to be careful to delete entries afterwards to avoid leaking memory.

Thoughts?

[sobolevn]

My suggestion would be to add this property to slots. How do you feel about adding test-specific behavior to non-test code?

Or we can create a runtime subclass of a container with this new slot 🤔
Something like type('_Pytest' + SomeType.__name__, (SomeType,), {'__slots__': ...}).

One other option would be to create a separate (<object id>, <object>): <is error handled> mapping in the test plugin

This also seems like a good idea.

[ariebovenberg]

I'll try the second approach first, as it should result in 'least surprise' for the user. 🤞 let's see if it works 👀