On Wed, Dec 1, 2021, 10:38 PM Steven D'Aprano 
"If param is missing **or None**, the default if blah..."
I reject Chris' characterisation of this as a hack. There are function parameters where None will *never* in any conceivable circumstances become a valid argument value, and it is safe to use it as a sentinel.

In particular, the cases where None will never, in any conceivable circumstances, be a non-sentinel value are at least 98% of all the functions (that have named parameters) I've ever written in Python.

All of this discussion of a syntax change is for, at most, 2% of functions that need a different sentinel.