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Python Performance: Why 'if not list' is 2x Faster Than Using len()
(blog.codingconfessions.com)
this post was submitted on 12 Apr 2025
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well it does not imply directly per se since you can "not" many things but I feel like my first assumption would be it is used in a bool context
I would say it depends heavily on the language. In Python, it's very common that different objects have some kind of Boolean interpretation, so assuming that an object is a bool because it is used in a Boolean context is a bit silly.
Well fair enough but I still like the fact that len makes the aim and the object more transparent on a quick look through the code which is what I am trying to get at. The supporting argument on bools wasn't't very to the point I agree.
That being said is there an application of "not" on other classes which cannot be replaced by some other more transparent operator (I confess I only know the bool and length context)? I would rather have transparently named operators rather than having to remember what "not" does on ten different types. I like duck typing as much as the next person, but when it is so opaque (name-wise) as in the case of "not", I prefer alternatives.
For instance having open or read on different objects which does really read or open some data vs not some object god knows what it does I should memorise each case.
Truthiness is so fundamental, in most languages, all values have a truthiness, whether they are bool or not. Even in C,
int x = value(); if (!x) x_is_not_zero();is valid and idiomatic.I appreciate the point that calling a method gives more context cues and potentially aids readability, but in this case I feel like
notis the python idiom people expect and reads just fine.I don't know, it throws me off but perhaps because I always use len in this context. Is there any generally applicable practical reason why one would prefer "not" over len? Is it just compactness and being pythonic?
It's very convenient not to have to remember a bunch of different means/methods for performing the same conceptual operation. You might call
len(x) == 0on a list, but next time it's a dict. Time after that it's a complex number. The next time it's an instance.notworks in all cases.I feel like that only serves the purpose up to the point that methods are not over reaching otherwise then it turns into remembering what a method does for a bunch of unrelated objects.
lenalso works on a dict.The point stands. If you want to check if a value is "empty," use the check for whether it's "empty." In Python, that's
not. If you care about different types of empty (e.g.Nonevs[]vs{}), then make those checks explicit. That reads a lot better than doing an explicit check where the more common "empty" check would be correct, and it also make it a lot more obvious when you're doing something special.