> I do not understand the problem with pickling sentinel values used as default values for function parameters. Could you please show an example?

A category of relevant uses I can think of is when wrapping a function and storing the parameters it was called with for some later use. Some examples of this are logging, caching and RPC.

Specifically for RPC, using pickle to serialize/deserialize the parameters and then call the function with them seems reasonable. RPyC[1] does this in some cases, though I haven't yet checked how it handles these kinds of sentinel objects specifically.

I'll try to find a good concrete example.

[1] https://rpyc.readthedocs.io/