Comment on Python Performance: Why 'if not list' is 2x Faster Than Using len()

• LegoBrickOnFire@lemmy.world ⁨2⁩ ⁨days⁩ ago

  Passing None to a function expecting a list is the error…

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• iAvicenna@lemmy.world ⁨2⁩ ⁨days⁩ ago

  good point I try to initialize None collections to empty collections in the beginning but not guaranteed and len would catch it

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  • sugar_in_your_tea@sh.itjust.works ⁨2⁩ ⁨days⁩ ago

    Sometimes there’s an important difference between None and []. That’s by far not the most common use, but it does exist (e.g. None could mean “user didn’t supply any data” and [] could mean “user explicitly supplied empty data”).

    If the distinction matters, make it explicit:

    if foo is None:
        raise ValueError("foo must be defined for this operation")
    if not foo:
        return None
    
    for bar in foo:
        ...
    
    return some_other_value
    

    This way you’re explicit about what constitutes an error vs no data, and the caller can differentiate as well. In most cases though, you don’t need that first check, if not foo can probably just return None or use some default value or whatever, and whether it’s None or [] doesn’t matter.

    if len(foo) == 0: is bad for a few reasons:

    • TypeError will be raised if it’s None, which is probably unexpected
    • it’s slower
    • it’s longer

    If you don’t care about the distinction, handle both the same way. If you do care, handle them separately.

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