PEP 412 makes __dict__s more memory efficient than they were before, but not more efficient than no __dict__, which is the point of __slots__. The following program demonstrates the difference. Note that it lowers the available address space to 1GB so that memory exhaustion occurs sooner, and thus only works on UNIX-like systems that provide the resource module.

  import resource
  import sys

  class WithoutSlots:
      def __init__(self, a, b):
          self.a = a
          self.b = b

  class WithSlots:
      __slots__ = ('a', 'b')
      def __init__(self, a, b):
          self.a = a
          self.b = b

  resource.setrlimit(resource.RLIMIT_AS, (1024 ** 3, 1024 ** 3))
  cls = WithSlots if sys.argv[1:] == ['slots'] else WithoutSlots
  count, instances = 0, []
  while True:
      try:
          instances.append(cls(1, 2))
      except MemoryError:
          break
  count = len(instances)
  del instances
  print(cls, count)

Here are numbers from my laptop:

  $ python3.6 /tmp/slots.py 
  <class '__main__.WithoutSlots'> 5830382
  $ python3.6 /tmp/slots.py slots
  <class '__main__.WithSlots'> 16081964

That's almost 3x more instances with __slots__! This isn't the case with PyPy, though, thanks to a more efficient representation of objects:

https://morepypy.blogspot.com/2010/11/efficiently-implementi...

That's a silly example. If you're making billions of integers, use NumPy. If it's just one pass, use a generator. If you're making lots of objects with more interesting attributes, the attribute storage will overwhelm the difference the instance dicts make.

My point was not that __slots__ does nothing, but that there are more important things to worry about.

Suppose I want to run algorithms on large arrays of 2D points while maximizing readability. I want to store the x and y coordinates using Python integers so I don't have to worry about overflow errors, but I expect that most of the time the numbers will be small and this is "just in case".

I claim that in this case, __slots__ is exactly the right thing to worry about.

It's hard for me to imagine that situation coming up, but yes, __slots__ does indeed have a purpose.

BTW, have you considered using the complex type to handle that for you? It's 2d and ints should be safe in float representation. If it overflows it'll crash nicely.

Good one. But let's say I want something mutable, so complex won't do.

That's an interesting example, and thanks for demonstrating it with a modern version! I definitely wouldn't have expected that result.