fluentpython/autoslot

Automatic __slots__ for your Python classes

autoslot

Automatic "__slots__".

Demo

from autoslot import Slots

class Compact(Slots):
    def __init__(self, a, b):
        self.x = a
        self.y = b

This produces exactly the same class as if you had done:

class Compact:
    __slots__ = {'x', 'y'}
    def __init__(self, a, b):
        self.x = a
        self.y = b

Simply: the code inside __init__() is scanned to find all assignments to attributes on self, and these are added as __slots__.

The benefit of using autoslot.Slots over a manual slots declaration is that you can modify the code inside the __init__() method to add more attributes, and those changes will automatically be reflected in the __slots__ definition.

You can also have the best of both worlds: slots for fields you expect, as well as a __dict__ for those you don't:

from autoslot import SlotsPlusDict

class SemiCompact(SlotsPlusDict):
    def __init__(self, a, b):
        self.x = a
        self.y = b

inst = SemiCompact(1, 2)
inst.z = 123  # <-- This won't fail!

Attributes x and y will be stored in slots, while all other dynamically-assigned attributes will go into the usual __dict__ instance inside the class. If most of your class's attributes appear in the __init__() method (these will become slots), then the space bloat caused by dictionary hash-table expansion will be contained to only the dynamically-assigned attributes.

How does it work?

See for yourself! The code is tiny.

In words: the metaclass finds the __init__() method, if present, and accesses its bytecode. It looks for all assignments to attributes of self, and considers those to be desired __slots__ entries. Then the metaclass injects __slots__ into the namespace of the class definition and thereafter allows class creation to proceed as normal.

Weakref

When __slots__ are used, weak references (e.g. using the weakref standard library module) won't work. If you need weak references, just set it up on a new __slots__ class variable as you would normally do without using autoslot:

from autoslot import Slots

class Compact(Slots):
    __slots__ = ['__weakref__']

    def __init__(self, a, b):
        self.x = a
        self.y = b

Everything else will still work, and instances of Compact will now also play nicely with the weakref module.