There is a very common pattern for creating optional arguments
when you can't use None:
_optional = object()
def foo(*, arg1='spam', arg3=None, arg4=_optional):
if arg4 is _optional:
# caller didn't pass *anything* for arg4
# caller did pass some (maybe None) value for arg4
It's a bit annoying to create this marker objects, and also,
if you try to render a signature of such function, you'll get
"(*, arg1='spam', arg3=None, arg4=<object object at 0x104be7080>)"
What if we add a standard marker for this use-case:
functools.optional or inspect.Parameter.optional?
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