On the closure front, you are assuming that `lambda` is the only way of writing a closure in Python. That is not true. You can write a named function (`def blah():`) in the same place, pass it around (within that scope the name is now `blah`), and it will still have access to the variables in the original scope. So it is a closure, just a named one.

At first I rebelled against not having unnamed closures, but then you hit the first problem in a complex case, and the backtrace having sane names suddenly makes it all more than worth it.