Python Slots Inheritance

admin  3/24/2022
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Slots

Contains iterable of instance data attributes which are only allowed to be accessed(set/get/delete).

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  • Instance methods, class methods, static methods, class variable are not affected by slots.
  • When slots are used __dict__ and __weakref__ is not created for objects. __dict__ and __weakref__ can be enabled by explicitly specifying in iterable passed to __slots__.
  • __slots__ uses less memory to store attributes and provides faster attribute lookup compared to __dict__.

How __slots__ uses less memory?

  • When using __dict__ python allocates new namespace dictionary to each instance of class, so that attribute can be assigned per instance base which can be expensive in terms of memory usage.
  • while using slots python shares single __slots__ attribute with all instances of class and when storing instance data attribute value fixed amount of memory is allocated as number of instance data attribute to be stored are fixed.

Inspecting __dict__ of Class and Object

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Defining slots and inspecting __slots__ of Class and Object

Output

  • when __slots__ is defined , Objects of class does not have the __dict__ attribute

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Inheritance
  • Setting instance data attribute not defined in __slots__

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__slots__ and __int__() method

If an instance data attribute name is not defined in __slots__, it can not be initialized using __init__ method.

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Slots with Instance method, Class Method, static Method and Class Variable

Only Instance data attribute are affected by __slots__, Instance methods, class methods, static methods, class variable are not affected by slots.

Output

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Using Different iterables to initialize Slots

Any iterable can be used to initialize __slots__ attribute except string.

Assigning tuple to __slots__

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Python Slots Inheritance Online

Assigning list to __slots__

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Assigning set to __slots__

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Assigning dict to __slots__

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Changing slots value after defining class

Python Class Inheritance List

Changing __slots__ value after defining class does not affect the Class definition, when classobject is created it creates class level descriptors for each instance data attribute which are present in iterable at the time of class definition creation.

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Slots attributes and Class attributes

Python uses Descriptors to define each attribute in slots iterable. So attribute name defined in slots can not be used as class variable name.

Python Inheritance Examples

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Using __dict__ with __slots__

'__dict__' can be added as an attribute to slots in order to enable attribute namespace dictionary.

Note : Using dict with slots requires extra memory compared to using only dict and lookup time may increase as now python needs to search for attribute in slots and dict.

Output

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Slots and Inheritance

  • Each class in single inheritance hierarchy can have their own slots and they are combined in single set from inheritance tree to check for validation when accessing attribute.
  • When inheriting class with __slots__, subclass __slots__ is not required to include attributes defined in superclass.

Output

  • Redefining the __slots__ attribute which are present in superclass will waste memory space.

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Slots and Multiple Inheritance

When using multiple inheritance only one class is allowed to have __slots__.

Multiple inheritance with more than one parent defining slots is not supported, and will result in TypeError: multiple bases have instance lay-out conflict.

Output

Subclass without __slots__ whose superclass is having __slots__

When Subclass does not defines __slots__ with superclass having __slots, superclass's __slots__ becomes class attribute to subclass and by default __dict__ and __weakref__ is enable for subclass.

Output

Subclass with __slots__ whose superclass does not have __slots__

Python Function Inheritance

When Subclass defines __slots__ and it's super class does not have __slots__ defined, __dict__ attribute of super class is inherited and use of __slots__ becomes meaningless.

Python Slots Inheritance Money

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