A common pattern, both in the stdlib and in user code, is the dedicated sentinel object to recognize an omitted argument: no_timeout = object() def get_data(source, timeout=no_timeout): if timeout is not no_timeout: source.set_alarm(timeout) return source.pop() This is a bit unclear in the function signature, as seen in help(): """ Help on function get_data in module __main__: get_data(source, timeout=<object object at 0x7f579fe53070>) """ The stdlib does this: """ Help on function create_connection in module socket: create_connection(address, timeout=<object object at 0x7fe52900b080>, source_address=None) """ # chomp docstring """ Help on function urlopen in module urllib.request: urlopen(url, data=None, timeout=<object object at 0x7fe52900b080>, *, cafile=None, capath=None, cadefault=False) """ # no docstring to chomp, actually It's not particularly useful to get the object's address. Proposal: A self-documenting Sentinel class which serves the exact same purpose. class Sentinel: def __init__(self, desc): self.desc = desc def __repr__(self): return "Sentinel(" + repr(self.desc) + ")" This can then be used just like object(), only it retains something for the benefit of its repr: no_timeout = Sentinel("No timeout") # Same function definition """ Help on function get_data in module __main__: get_data(source, timeout=Sentinel('No timeout')) """ I don't know how this interacts with Argument Clinic and C-written functions. If there's something that's being done for those that would make sentinels come out a particular way in their help() info, ideally this should be displayed the same way (or at least similarly). Thoughts? ChrisA