While I love this idea, and I think this PEP is bound for glory, I don't love any of the proposed spellings for denoting late-binding arguments. It seems to me that a specialized symbol combination indicating that a particular argument has special behavior does not serve readability, regardless of what symbol combination is chosen.I would prefer to build on the fact that arguments already come in two flavors with somewhat different behaviors, and that the ordering of these is determined. Going by this analogy, it would make sense to have late-binding arguments following keyword arguments, set off by some separator, such as a double-pipe:
def my_func(positional_arg, some_kwarg="foo" || late_binding_arg=[]]:
# body of func
or
def my_func(positional_arg, some_kwarg="foo" ||
late_binding_arg=[]):
# body of func
I'm not massively attached to this particular symbol (which has advantages and disadvantages), but more to the idea that we can easily see that the arguments to the right of it are late-binding, and we can easily teach a beginner what that means.
We can also easily scan a function definition and know that there is some specialized behavior going on, which is harder to see with a spelling like
def my_func(positional_arg, some_kwarg<="foo",
another_kwarg=[]):
# body of func
and we will probably even notice much more easily that there is a likely bug in that last definition (since we probably meant for the list arg to be late-binding)
Best
-Jon Kiparsky