can't add variables to instances of built-in classes

[Lawrence D’Oliveiro]

On Tuesday, July 19, 2016 at 9:48:15 AM UTC+12, I wrote:

> When you have lots of read/write properties, I find __slots__ to be a good
> idea.

Let me amend that. When you have *any* read/write properties, I find __slots__ to be a good idea.