python-patterns

e concept of a global variable. Globals are great. I love them. But they require discipline. If you want to spin up a bunch of threads that each have their own view of that global, you’re hosed. Except you’re not. What you can do is have a thread local variable that initializes itself to the value of the global variable. That way new threads start with the current value of the global. Why? Because suddenly you can just pretend like you’re using globals everywhere! Whenever you want to dish out some work, set the global to foo, then spin up a thread. Set the global to bar, then spin up another thread. Instead of passing that damn value through 100 smaller functions, all the code simply refers to “the current user” or “the current webpage” or “the current widget”, which is a global concept. If you ever need to modify it, you set the global and spin up another thread. Internally it’s implemented by a pattern similar to borg. The instance returns its thread local state, unless it’s nil, in which case it initializes from the global value. The nitpick is that this needs to be true for child threads that spawn threads, otherwise the whole thing doesn’t work well anymore. Racket nailed this concept with thread cells. If a thread cell is preserved, any child thread immediately inherits the value from the spawning thread. And it’s all transparent. With this borg example, it’s just literally a global variable. Or maybe it’s not and someone will come say why that’s mistaken. But I don’t think so!