Python | Use of __slots__

Python | Use of __slots__

In Python, the __slots__ attribute has a special meaning for classes. It allows for the explicit declaration of instance attributes and can help in reducing the memory footprint of objects, especially when creating many instances of a class.

Basics of __slots__:

  1. Memory Optimization: In Python, every object has a dictionary (__dict__) to hold its attributes. Dictionaries consume a considerable amount of memory due to their underlying hash table implementation. __slots__ allows us to avoid using this dictionary, thereby saving memory.

  2. Attribute Restriction: Once __slots__ is defined for a class, it restricts the class from creating any new attributes outside those defined in __slots__. This can help in preventing accidental creation of new attributes.

Usage:

Here's a simple class definition without __slots__:

class WithoutSlots: def __init__(self, name, age): self.name = name self.age = age

With __slots__:

class WithSlots: __slots__ = ('name', 'age') def __init__(self, name, age): self.name = name self.age = age

Points to Remember:

  1. Inheritance: Derived classes inherit the __slots__ of their base classes. If a derived class also defines __slots__, then the set of allowed attributes is a combination of the base and derived __slots__.

  2. Overhead: __slots__ introduces its own overhead. So, the memory benefits are more pronounced when you have a large number of small objects.

  3. Restricted Dynamic Attributes: Once __slots__ is used, you can't add new attributes to the object outside of those defined in __slots__:

    obj = WithSlots('John', 25) obj.address = 'Some place' # Raises AttributeError

  4. Still has other standard attributes: While the object may not have a __dict__ (unless '__dict__' is specifically included in __slots__), it still has attributes like __class__, __slots__, and the methods defined for the class.

  5. Descriptor-Based Attributes: __slots__ uses descriptors internally. If you define a slot with the same name as a method or property, the slot will overshadow the method or property, potentially breaking the class's behavior.

  6. Not always the best choice: __slots__ isn't a silver bullet. It's useful for specific cases where memory optimization is crucial. For general purposes, the standard object model without __slots__ offers more flexibility.

In summary, __slots__ is a powerful tool for optimizing memory and controlling attribute assignments in specific scenarios, but it should be used judiciously.

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