Slots in python dict

Posted on March 1, 2020  •  One minute  • 158 words

The special attribute __slots__ allows you to explicitly state which instance attributes you expect your object instances to have. The benefits of using __slots__ are as follows:

• faster attribute access.
• space savings in memory.

The space savings is from the following:

• Storing value references in slots instead of __dict__
• Denying __dict__ and __weakref__ creation if parent classes deny them and you declare __slots__

Example:

  class Person(object):
    __slots__ = ('a', 'b')

Now, you can’t have dynamic properties in person class ie. You can only access and modify properties a and b.

  person = Person()
  person.a = 'a'
  person.b = 'b'
  person.c = 'c'.  -> gives error

Requirements:

• To have attributes named in __slots__ to actually be stored in slots instead of a __dict__, a class must inherit from object.
• To prevent the creation of a __dict__, you must inherit from object and all classes in the inheritance must declare __slots__ and none of them can have a __dict__ entry.