Signatures, a call to action

[CAM-Gerlach]

@cameron

We tend to use None because it is usually a good sentinel - it
isn’t in the valid domain for the parameter; here base.

Indeed, and I in fact just made the same point over on the other thread before I’d read this one:

Should `None` defaults for optional arguments be discouraged?

This can work both ways, though, as even if something is syntactically a float, int, etc (like using 0, -1, or even math.e for a flag value, as some has proposed), semantically the meaning and behavior is a special case, different from that with any other int/float. Using None (or a dedicated sentinel type) ensures the syntax clearly communicates that this is a semantic special case, and can allows type checkers and other tools to statically verify it is being handled inside the function (or wra…