this post was submitted on 24 Apr 2025
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Python Performance: Why 'if not list' is 2x Faster Than Using len() (blog.codingconfessions.com)
submitted 2 months ago by [email protected] to c/[email protected]
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[–] [email protected] 13 points 2 months ago (5 children)

I think the explicitness of checking length is worth the performance cost. If ur writing code for speed ur not using python.

[–] [email protected] 18 points 2 months ago (1 children)

I'd argue that if it's strict explicitness you want, python is the wrong language. if not var is a standard pattern in python. You would be making your code slower for no good reason.

[–] [email protected] 10 points 2 months ago (2 children)

You always want explicitness when programming. Not everyone reading your code will be deep into Python and relying on falsiness makes it harder to understand.

[–] [email protected] 5 points 2 months ago (1 children)

While I agree to some extent, if not var is more than clear enough for anyone that knows python. If that pattern confuses someone, they probably aren't at level where they should be dealing with python at a professional level. The same way you would expect people to understand pointers and references before delving into C code.

This sort of stuff is something I taught first year engineering student in their programming introductory course, it's just how python is written.

For what it's worth, it's sort of similar in Erlang/Elixir. If you want to check if a list is empty, checking the length of the list is discouraged. Instead you would use Enum.empty?().

[–] [email protected] 2 points 2 months ago (1 children)

Well, it certainly wouldn't be the first time that I call some code unnecessarily hard to read and others call it pythonic.

I understand that truthiness has an advantage when you don't have static types, because it deals somewhat reasonably with most types, and then that is why many experienced Python programmers tend to use it. But I maintain the position that it therefore necessarily also makes it harder to read the code, because you necessarily provide the reader with fewer hints as to what is actually in that variable. It is very much like code using lots of generics in statically typed code.

As for if Enum.empty?(var), I actually prefer that to checking the length. That syntax in particular, I find a bit too complex – my preferred version is if var.is_empty() – but I still think it makes it easier to read than a length check.
Of course, there's the nuance that it's more syntax to remember for actually writing the code. And in particular in dynamically typed languages, your editor may not be able to auto-complete that, so I can understand just not bothering with the extra syntax and doing len == 0 instead. It's fine.

[–] sugar_in_your_tea 1 points 2 months ago

you necessarily provide the reader with fewer hints as to what is actually in that variable

Then make it explicit. I much prefer this:

def do_foo(bar: list | None):
    if not bar:
        return
    ...

This one communicates to me that the function only makes sense with a non-empty list.

To this:

def do_foo(bar):
    if len(bar) == 0:
        return

This just tells me bar is something that has a length (list, dict, str, etc).

And this is way worse:

def do_foo(bar: list | None):
    if len(bar) == 0:
        return

This tells me we want an exception if bar is None, which I think is bad style, given that we're explicitly allowing None here. If we remove the | None and get an exception, than the code is broken because I shouldn't be able to get that value in that context.

If I care about the difference between None and empty, then I'll make that explicit:

if bar is None:
    ...
if not bar:
    ...

In any case, I just do not like if len(bar) == 0 because that looks like a mistake (i.e. did the dev know it could throw an error if it's not a list? Was that intentional?).

[–] [email protected] 2 points 2 months ago (1 children)

Containers being "truthy" is quite basic Python and you will find this idiom used in nearly every Python code base in my experience

[–] [email protected] 3 points 2 months ago* (last edited 2 months ago) (1 children)

Yeah, I'm talking less deep than that. Plenty programming beginners will be reading Python code. And personally, I'm a fulltime software engineer, but just don't do much Python, so while I had it in the back of my mind that Python does truthiness, I would have still thought that var must be a boolean, because it's being negated. Obviously, a different variable name might've given me more of a clue, but it really doesn't reduce mental complexity when I can't be sure what's actually in a variable.

[–] [email protected] 2 points 2 months ago* (last edited 2 months ago) (1 children)

But if those beginners want to stop being beginners, then they must learn the basics of the language. It makes no more sense to demand that everyone who programs in Python caters to beginners, than it makes to demand that everyone writing in English write at a 3rd grade reading level for the sake of English language learners

[–] [email protected] 3 points 2 months ago

Yeah, my stance for both of those is the same: If the complexity aids you in communicating better, then use it. But if you're using big words where small words would do, then you're doing a disservice to your readers.

[–] [email protected] 4 points 2 months ago (2 children)

I never understood that argument. If you can be sure the type is a collection (and this you always should) not list is so moch easier to read and understood than the length check.

[–] [email protected] 4 points 2 months ago* (last edited 2 months ago)

How many elements in that list? Ah, it's not list. It's list, of course, we checked. But it's not.

[–] [email protected] 2 points 2 months ago (1 children)

Is it? Why introduce an additional conversion from not list means empty list that u have to hold in your head. I want to check length I check length I would argue is easyer to comprehend.

[–] sugar_in_your_tea 2 points 2 months ago (1 children)

Because that's a fundamental aspect of Python. When you're using a language, you should be familiar with the truthiness values. In Python, it's pretty sane:

  • [], {}, set(), "", None, False 0 and related values are all "falesy"
  • everything else is truthy

Basically, if you have non-default values, it's truthy. Why wouldn't you trust basic features of the language?

[–] [email protected] 1 points 2 months ago (1 children)

Because I have to do the is this falsy to what I'm actually interested conversion in my head.

Say ur deep inside some complicated piece of logic and u are trying to understand. Now u have a bunch of assumptions in your head. You should be trying to eliminate as many if these assumptions with good code as possible eg u test ur fail case and return/continue that so u don't need to keep that assumption in ur head.

Say I then come along a if not x then you have to figure out what is x what is the truthiness of its type. If I come across an if len(x) == 0 then I automatically know that x is some collection of objects and I'm testing its emptiness.

[–] sugar_in_your_tea 0 points 2 months ago (1 children)

That's why there's type hinting, unit tests, and doc strings. I don't need to guess what the type is intended to be, I can see it.

[–] [email protected] 1 points 2 months ago (1 children)

But that's an extra step of logic u must hold in ur head while trying to understand 12 other things.

[–] sugar_in_your_tea 1 points 2 months ago

What's the extra logic?

if x:

This always evaluates to True if it's non-empty. There's no extra logic.

If you have to keep 12 things in your head, your code is poorly structured/documented. A given function should be simple, making it plainly obvious what it's intended to do. Use type hints to specify what a variable should be, and use static analysis to catch most deviations. The more you trust your tools, the more assumptions you can safely make.

[–] sugar_in_your_tea 4 points 2 months ago (1 children)

Why? not x means x is None or len(x) == 0 for lists. len(x) == 0 will raise an exception if x is None. In most cases, the distinction between None and [] isn't important, and if it is, I'd expect separate checks for those (again, for explicitness) since you'd presumably handle each case differently.

In short:

  • if the distinction between None and [] is important, have separate checks
  • if not, not x should be your default, since that way it's a common pattern for all types
[–] [email protected] 1 points 2 months ago (1 children)

I try to avoid having the same variable with different types I find it is often the cause of difficult to debug bugs. I struggle to think of a case where u would be performing a check that could be an empty list or None where both are expected possible values.

[–] sugar_in_your_tea 2 points 2 months ago* (last edited 2 months ago) (1 children)

Really? I get that all the time. I do web dev, and our APIs have a lot of optional fields.

[–] [email protected] 0 points 2 months ago (1 children)

I do web dev

Theirs ur problem.

But in all seriousness I think if u def some_func(*args, kwarg=[]) Is a more explicit form of def some_func(*args, kwarg=None)

[–] sugar_in_your_tea 3 points 2 months ago* (last edited 2 months ago) (1 children)

def some_func(*args, kwarg=[])

Don't do this:

def fun(l=[]):
    l.append(len(l))
    return l

fun()  # [0]
fun()  # [0, 1]
fun(l=[])  # [0]
fun()  # [0, 1, 2]
fun(l=None)  # raise AttributeError or TypeError if len(l) comes first

This can be downright cryptic if you're passing things dynamically, such as:

def caller(*args, **kwargs):
    fun(*args, **kwargs)

It's much safer to do a simple check at the beginning:

if not l: 
    l = []
[–] [email protected] 1 points 2 months ago (2 children)

I like the exception being raised their is no reason I should be passing in None to the function it means I've fucked up the value of whatever I'm passing in at some point.

[–] sugar_in_your_tea 2 points 2 months ago

Then make it explicit:

if l is None:
    raise ValueError("Must provide a valid value for...")

Having an attribute or type error rarely provides the right amount of context to immediately recognize the error, especially if it's deep inside the application. A lot of our old code makes stupid errors like TypeError: operator - not defined on types NoneType and float, because someone screwed up somewhere and wasn't strict on checks. Don't reply on implicit exceptions, explicitly raise them so you can add context, because sometimes stacktraces get lost and all you have is the error message.

But in my experience, the practical difference between [] and None is essentially zero, except in a few cases, and those should stand out. I have a few places with logic like this:

if l is None:
    raise MyCustomInvalidException("Must provide a list")
if not l: 
    # nothing to do
    return

For example, if I make a task runner, an empty list could validly mean no arguments, while a null list means the caller screwed up somewhere and probably forgot to provide them.

Explicit is better than implicit, and simple is better than complex.

[–] [email protected] 2 points 2 months ago (1 children)

Oh no a stray None! Take cover ...

Robust codebase should never fail from a stray None

Chaos testing is specifically geared towards bullet proofing code against unexpected param types including None.

The only exception is for private support function for type specific checking functions. Where it's obviously only for one type ever.

We live in clownworld, i'm a clown and keep the company of shit throwing monkeys.

[–] [email protected] 1 points 2 months ago (1 children)

Ur function args if fucked up should always throw an error that's the entire point of python type hints

[–] [email protected] 1 points 2 months ago

type hints are static, not necessarily runtime.

A chaos monkey throws everything at everything to see what breaks.

That won't be caught by perfect type hints, which is merely one tool in the toolbox.

and when things break, often hear WAD, works as designed. Or some other nonsense excuse.

[–] [email protected] 3 points 2 months ago* (last edited 2 months ago)

In complex cases where speed is less important than maintainability, I tend to agree.

In this case, a simple comment would suffice. And in fact nothing at all would be okay for any half-competent Python coder, as testing if lists are empty with if not is super-standard.

[–] [email protected] 1 points 2 months ago

Another use case is to reduce maintenance cost