abhi9uPython Performance: Why 'if not list' is 2x Faster Than Using len()
AvicennaYea and then you use “not” with a variable name that does not make it obvious that it is a list and another person who reads the code thinks it is a bool. Hell a couple of months later you yourself wont even understand that it is a list. You should not sacrifice code readability for over optimization, this is phyton after all I don’t think list lengths will be your bottle neck.
In my experience, if you didn’t write the function that creates the list, there’s a solid chance it could be
None too, and if you try to check the length of None, you get an error. This is also why returning None when a function fails is bad practice IMO, but that doesn’t seem to stop my coworkers.
good point I try to initialize None collections to empty collections in the beginning but not guaranteed and len would catch it
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_valueThis 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:
TypeErrorwill be raised if it’sNone, 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.