What is the goal of __slots__ in Python?

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What is the purpose of __slots__ in Python. Especially regarding when to use it and when not to?

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TLDR:

The special attribute __slots__ allows you to explicitly specify which instance attributes you expect from instances of your object, with the expected results.:

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1. quick access to attributes.
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2. saving memory.
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Space is saved because

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   References are saved to the value of slots and not in __dict__

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   Prohibiting the creation of __dict__ and __weakref__, if the parent classes forbid them, and you declare __slots__.

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Brief waings

In the inheritance tree, you only need to declare a specific slot once

For example:

class/span> Base:n    __slots__ = 'foo', 'bar'nclass/span>nclass">class Right(Base):n    __slots__ = 'baz',n    nclass/span> Wrong">Wrong(Base):n    __slots__ = 'foo', 'bar', 'baz' # redundant foo and barn

Python doesn't mind when you make mistakes (it probably should), otherwise the problems may not show up, but your objectsobjects will take up more space than they should.

Python 3.8:

>>> from sys import getsizeofn>n>>> getsizeof(Right()), getsizeof(Wrong())n(56, 7272) n

This is because the Base slot descriptor has a slot separate from Wrong.

>>;>> w = Wrong()n>>> w.foo = 'foo'n>>> Base.foo.__get__(w)nTraceback (most recent call last):n  File ""<stdin>"", line 1, in <modulein <module>nAttributeError: foon>>> Wrong.foo.__get__(w)n'foo''foo'n

The biggest caveat conces multiple inheritance - multiple "parent classes with non-empty slots"; they cannot be combined.

To circumvent this limitation, follow the recommendations: Isolate all abstractions of the parents, except for one or all of which their specific class will inherit from, and your new class by giving the abstractions empty slots (just like the abstract base classes in the standard library)

See See the example below in the section on multiple inheritance.

Requirements:

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   So that attributes named in __slots__ are stored in slots and not in __dict__, class must inherit from object.

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   To prevent the creation of __dict__, you must inherit from objectw, and all classes in inheritance must declare __slots__, and none of them can have an entry __dict__.

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If you want to continue reading, there are many more details.

Why use __slots__: faster access to attributes.

The creator of Python, Guido van Rossum, says that he actually created __slots__ for faster access to attributes.

Demonstrate faster access - this is a trivial task:

import timeitnclass>class Foo(object): __slots__ = 'foo',n,nclass Bar(object): passnslotted = Fonslotted = Foo()nnot_slotted = Bted = Bar()ndef get_set_delete_fn(obj):n    def/span> get_set_delete():n      &nbssp;  obj.foo = 'foo'n                                 del obj.foon retu get_set_deleten

and

>>> min(timeit.repeat(get_set_delete_fn(slotted)))n0.2846834 class="">0.2846834529991611n>>> min(timeit.repeat(get_set_delete_fn(not_slotted)))n0.3664822799983085 n