Python __slots__ for Compact, Safer and Faster Code

Posted by amitosh on — 1 min read...

Setting __slots__ in your Python class can have significant gains in speed and memory usage.

Python classes consists a magic attribute called __slots__ this defines the attributes that an instance of the defining class can hold.

Python is a dynamic language so normally you can set any attribute to any object as opposed to a static language like Java where you need to define a field explicitly before setting any value. But this dynamic behaviour comes at a cost. All Python objects are essentially a wrapper over a dict (the __dict__ attribute). This makes attribute access a lot slower and since we hold additional baggage capacity, it makes object sizes grow to enormous amounts.

By defining slots you trade the dynamic behaviour for much faster access and smaller memory requirements. Attributes are now directly stored in dedicated slots instead of __dict__ and can be directly accessed.

class Product(object):
__slots__ = ['name', 'price']
def __init(self, name:str, price:int): = name
self.price = price
# ...

Let the lack of dynamic capabilities not scare you. You are already writing good code right? You should ideally not be adding attributes to objects during runtime. A majority of the time this is true.

If you are using mypy or pyre then using slots will enforce an additional layer of safety.


Noting is free

Slots, other than eliminating dynamic attributes, also make it impossible for you to perform multiple inheritances without any of the parents having free slots.

Written By
Amitosh Swain Mahapatra

Developer, computer whisperer

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