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 wrappers) and better introspect usage outside of it.
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