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Much Ado About None: Sentinel Values in Python

Much Ado About None: Sentinel Values in Python

sebg

Comments (38)

  • pwdisswordfishq
    So much trouble to avoid introducing the optional monad.
  • lpghatguy
    When I was writing a bunch of Lua, I was a big fan of having a None sentinel value (Lua uses 'nil' for representing absence) and it was useful for a lot of things similar to Pydantic's MISSING.This distinction is something that I think is actually handy about JavaScript, and distinguishing between 'null' and 'undefined' is sometimes useful for similar reasons.
  • RantyDave
    "A simple cache returns None both for a missing key and for a key that maps to None."No, it returns KeyError. Or at least it should.
  • est
    I just return a customized Exception type. Works about the same.
  • chowells
    I like it. I can't see how to do better while remaining compatible with Python idioms.
  • throwaway2037
    Here is the sample code from the article: > MISSING = sentinel("MISSING") > def update_email(user_id: int, email: str | None | MISSING = MISSING) -> None: > if email is MISSING: > return # nothing to update > elif email is None: > db.delete_email(user_id) > else: > db.set_email(user_id, email.lower()) # narrowed to str I see this style frequently from long time Python programmers. The language is so dynamic that people begin to think this is a good way to program. Hot take: It is not. Why not two different functions called delete_email() and set_email()? That would be more clear.
  • jMyles
    I have a library that's approximately half joking that I use for (sentinel and other) special value constants. If you like reading test suites, this one will get ya:https://github.com/nucypher/constantSorrow/blob/master/tests...