Discover why 'if not mylist' is twice as fast as 'len(mylist) == 0' by examining CPython's VM instructions and object memory access patterns.
    • sugar_in_your_tea@sh.itjust.works
      0·
      6 days ago

      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
            • sugar_in_your_tea@sh.itjust.works
              0·
              5 days ago

              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 = []
                • logging_strict@programming.dev
                  0·
                  4 days ago

                  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.

    • ExtremeDullard@lemmy.sdf.org
      0·
      6 days 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.