What Counts as Empty, False, or Equal

Write if some_list: instead of if len(some_list) > 0: and Python will do exactly what you mean — but only because it treats every empty collection, and the number zero, as false.

The list is short, and the trap is what is missing from it. The string "False" is not empty — it has five characters in it — so it is truthy, despite spelling out the word. A list holding a single 0 is not empty either; it has one item, so it too is truthy.

$ >>> bool(0)$ False$ >>> bool("False")$ True$ >>> bool([])$ False$ >>> bool([0])$ True

if x: treats an empty list, the number 0, and None as exactly the same kind of false. Most of the time that is fine. It stops being fine the moment an empty list or a zero is a valid answer you still need to tell apart from no answer having arrived at all.

$ >>> def summarise(votes=None):$ ...     if votes:$ ...         print(f"{len(votes)} votes counted")$ ...     else:$ ...         print("no votes yet")$ ...$ >>> summarise([])$ no votes yet$ >>> summarise([1, 2, 3])$ 3 votes counted

The round with zero votes and the round that has not started yet print the identical message, because [] and None both fail if votes:. Ask the narrower question instead, and the two cases separate:

$ >>> def summarise(votes=None):$ ...     if votes is not None:$ ...         print(f"{len(votes)} votes counted")$ ...     else:$ ...         print("no votes yet")$ ...$ >>> summarise([])$ 0 votes counted

== asks whether two things hold equal values. is asks whether two names point at the exact same object sitting in memory. Almost every comparison you write wants ==.

$ >>> a = [1, 2, 3]$ >>> b = [1, 2, 3]$ >>> a == b$ True$ >>> a is b$ False

a and b are two separate lists that happen to hold equal contents, built on two separate lines. is sees straight through the equal values to the fact that they are different objects — change a, and b does not move.

The one place is is the right tool, always, is comparing against None. None is a single object that exists exactly once for the whole program, so identity is precisely the question you mean to ask.

$ >>> x = None$ >>> x is None$ True

A set is an unordered collection that will not hold the same value twice — add a duplicate, and nothing happens, silently.

$ >>> tags = ["python", "beginner", "python", "tutorial"]$ >>> set(tags)$ {'python', 'beginner', 'tutorial'}

Turning a list into a set is the fastest way to deduplicate it. Notice the order is not preserved — a set does not remember which item came first, only which items exist. If you need the duplicates gone and the original order kept, a dictionary does the job instead, because a dictionary's keys can never repeat and it does remember insertion order:

$ >>> list(dict.fromkeys(tags))$ ['python', 'beginner', 'tutorial']

A set is not just a list with duplicates removed. It carries its own arithmetic for comparing two collections at once, straight out of how you would describe sets on a maths whiteboard.

$ >>> admins = {"ada", "grace"}$ >>> editors = {"grace", "alan"}$ >>> admins | editors$ {'ada', 'grace', 'alan'}$ >>> admins & editors$ {'grace'}$ >>> admins - editors$ {'ada'}$ >>> admins ^ editors$ {'ada', 'alan'}

admins & editors answers "who has both roles" in one expression, with no loop and no temporary list — which is usually why a set was the right choice before deduplication even came into it.

Membership testing is the reason sets exist at all. Checking whether a value is in a list means Python walks the list from the start until it finds it, or reaches the end. Checking a set is close to instant regardless of size, because a set is built, internally, for exactly this question.

$ >>> allowed = {"admin", "editor", "viewer"}$ >>> "editor" in allowed$ True