Python Performance: Why 'if not list' is 2x Faster Than Using len()
Python Performance: Why 'if not list' is 2x Faster Than Using len()
Discover why 'if not mylist' is twice as fast as 'len(mylist) == 0' by examining CPython's VM instructions and object memory access patterns.
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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?
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)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.
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.
else: # not my list, it is ourlist
Python likes giving lists.