Using __slots__ to Store Instance’s Attributes

Typically all attributes are stored in the instance’s dictionary __dict__. This will add space overhead due to the hash table being sparse. We can save memory by storing all the attributes in a tuple-like data structure using __slots__ at the beginning of the class definition (see below):

import sys


class C:
    def __init__(self, x, y):
        self.x = x
        self.y = y

class C_Slots:
    __slots__ = ("x", "y")

    def __init__(self, x, y):
        self.x = x
        self.y = y

print(sys.getsizeof(C))         #=> 904
print(sys.getsizeof(C_Slots))   #=> 1,072

The main problems with __slots__:

• Can’t dynamically add attributes to the instance. We can work around this issue by adding __dict__ to __slots__ but this negates the benefits of using __slots__.
• Doesn’t support __weakref__ by default. We need to add __weakref__ to __slots__ to be able to use it.
• All sublcasses have to declare __slots__ because attributes can’t be inheritted and will be ignored by the interpreter.

In conclusion, use __slots__ only if it is justified.