Having mylist stick around is actually useful for making stateful functions. For example, I've often used

  def foo(..., cache={}):
    ...

to implement cached functions, or functions which need an internal cache for whatever reason. Later versions of Python provide an lru cache decorator, but that doesn't help with the internal cache and can be inefficient when you don't want your cache to forget any results.

Yeah, the more I look at it, the more I'm a fan of "?=". Kind of seems like the obvious choice at this point.

The only potential downside is that standard, function definition-time default arguments also imply "present? else = ...". But I think of all the options available, this one makes the most sense.

What about .&= for default arguments?, inspired by Ruby.