No, you can implement the slots in pure Python if you’d like. You’ll be wasting a perfectly good dictionary on each object, but that’s still less processing overhead.

What slots do is they essentially create descriptors for each attribute named. For example: http://pb.lericson.se/p/SkRPZb/

Descriptors are used for implementing static methods, class methods, unbound/bound methods, and properties. They’re very essential to Python’s OOP. Nothing weird.

This ability to add slots allows one to put in optimizations if needed, but keeps the default case simple and flexible. I like that.

class Foo(object):
__slots__ = ['x']
def __init__(self, n):
self.x = n

y = Foo(1)
print y.x # prints “1″
y.x = 2
print y.x # prints “2″
y.z = 4 # Throws exception.
print y.z

Cheers though! Very helpful!

It’s also worth note that slots layer up – a little confusingly at first, but quite understandably. You can define a class X(slots=a) and subclass Y(slots=b) and things will work as expected. I have no idea how memory is allocated for that underneath. Perhaps Python’s type object is smart enough to merge them into a single object.

The other last thing worth note is that slots can have a very positive impact on performance, not just on memory usage.

Great post,

David (dwdwdw@Reddit!)