Consider this:

def defaultKeepsIdentity(arg = "str_value"):
    print(arg is "str_value")

defaultKeepsIdentity()

This has been outputing "True" on every Python release I have seen so far, but not so on 3.6.0a4. Normally string values come from a "co_const" and will be "is" identical if used as literals in a module, but no longer in this case.

This seems wasteful at best, needlessly increasing the number of strings in usage. 

Yours,
Kay