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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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How does Python know if it's my list or not?
Telemetry
if isinstance(mylist, list) and not mylistProblem solved.
Or
if not mylist # check if list is emptyI think you missed the joke 😅
I thought it was funny!
You’re checking if
mylistis falsey. Sometimes that’s the same as checking if it’s empty, if it’s actually a list, but that’s not guaranteed.Doesn't Python treat all empty iterables as false tho? This isn't unique to python, is it? (though I'm not a programmer...just a dude who writes scripts every now and then)
My point is that the second statement you presented can have the effect of evaluating emptiness of a Sequence (note: distinct from an Iterable), but that only holds true if the target of the conditional IS a sequence. I’m underlining the semantic difference that was elided as a result of falsey evaluation.
Ok, help a noob out. What is the difference between a sequence and an iterable? Is a sequence immutable, like a tuple?
An iterable is just something that can be iterated over, like
range(10), or[1, 2, 3].A sequence on the other hand is a Collection that is reversible.
https://docs.python.org/3/library/collections.abc.html#collections-abstract-base-classes
I know what an iterable is. But I am talking about
Type[Iterable], which iirc does not obey falsey eval when empty.thing: Sequence[Any]iirc is iterable, indexable, and reversible.thing: Iterable[Any]only guarantees that its iterable - and note that iterating can sometimes have the effect of consuming the iterable (e.g. when working with streaming interfaces)Not really, generators have weird truthiness, i don't remember if they evaluate to true or false, but they cannot be checked for emptiness so they default to either always true or always false.
Python likes giving lists.
else: # not my list, it is ourlist