[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...

[xapata]

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.

[marvy]

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.

[xapata]

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.

[marvy]

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

[kstrauser]

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