autoslot 2017.10.6

Classes and metaclasses for easier ``__slots__`` handling.

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 the metaclass version 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

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 fields are expected, then dictionary bloat will be contained.

How does it work?

See for yourself–the code is embarrassingly small!

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.

Note

If __init__ is missing, the class will not have __slots__ injected. It will be a normal class with a __dict__ attribute.