TBH I think it's usually an anti-pattern when you have an API where the absence of a parameter is not equivalent to some default value. It makes wrapping such APIs awkward, e.g.

def logging_foo(*, arg1='something_other_than_spam', arg3=None, arg4=<what???>):
    loging.info('calling foo(arg1=%r, arg3=%r, arg4=%r)', arg1, arg3, arg4)
    return foo(arg1=arg1, arg3=arg3, arg4=arg4)

On Wed, Apr 16, 2014 at 10:09 AM, Yury Selivanov <yselivanov.ml@gmail.com> wrote:

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
something like:

"(*, 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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