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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.
I definitely agree that
lenis the preferred choice for checking the emptiness of an object, for the reasons you mention. I'm just pointing out that assuming a variable is a bool because it's used in a Boolean context is a bit silly, especially in Python or other languages where any object can have a truthiness value, and where this is commonly utilised.It is not "assume" as in a conscious "this is probably a bool I will assume so" but more like a slip of attention by someone who is more used to the bool context of not. Is "not integer" or "not list" really that commonly used that it is even comparable to its usage in bool context?
Then I absolutely understand you :)
How common it is 100 % depends on the code base and what practices are preferred. In Python code bases where I have a word in decisions, all Boolean checks should be
x is Trueorx is Falseifxshould be a Boolean. In that sense, if I readif xorif not x, it's an indicator thatxdoes not need to be a Boolean.In that sense, I could say that my preference is to flip it (in Python): Explicitly indicate/check for a Boolean if you expect/need a Boolean, otherwise use a "truethiness" check.