Comment by markrwilliams

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...