Ordered Collections, Two Ways

A list holds items in order, and you can add to it, remove from it, or replace an item at a position, all after it already exists.

$ >>> fruit = ["apple", "banana"]$ >>> fruit.append("cherry")$ >>> fruit$ ['apple', 'banana', 'cherry']$ >>> fruit[0] = "avocado"$ >>> fruit$ ['avocado', 'banana', 'cherry']

Nothing requires the items in a list to share a type — [1, "two", 3.0] is a perfectly ordinary list of three items, an int, a string, and a float, all in the same one. Python does not check or care; it only cares that the list itself stays ordered and stays a list.

A tuple looks almost identical — ordered, indexable, written with () instead of [] — except that once it is created, it cannot be changed at all.

A tuple being locked is not a missing feature, it is the point. A function that returns a pair of coordinates, (x, y), returns a tuple on purpose — nothing downstream can accidentally reorder or extend it, because nothing downstream is allowed to touch it at all.

As a rule of thumb: reach for a tuple when the number of items is fixed by what the data is — a coordinate is always two numbers, a date is always three. Reach for a list when the number of items is expected to change while the program runs.

You already know from the earlier lesson on variables that a name points at a value, it does not hold a copy of one. Lists make the consequence hard to miss: assigning one list to a second name never copies it, no matter how much the assignment looks like it should.

$ >>> original = [1, 2, 3]$ >>> copy = original$ >>> copy.append(4)$ >>> original$ [1, 2, 3, 4]

copy was never a copy. It is a second name on the same list, so changing it through one name is visible through the other. The same trap reappears the moment you hand a list to a function — the parameter inside the function is another name on your original list, not a private version of it.

def add_bonus(scores):    scores.append(100) results = [88, 92, 79]add_bonus(results)print(results)

Try itRun it, then change the call to add_bonus(results.copy()) and run it again.

Nothing about add_bonus looks dangerous, and the effect on results was not written anywhere in the calling code. If you meant to work on a private copy, you have to ask for one explicitly — there are three equivalent ways.

$ >>> original = [1, 2, 3]$ >>> safe = original.copy()$ >>> safe = list(original)$ >>> safe = original[:]$ >>> safe.append(4)$ >>> original$ [1, 2, 3]

A list comes with a handful of methods that change it in place. Most of them hand you something useful back as well — except the one every beginner eventually assigns away by accident.

The safe alternative already has a different name, on purpose: the built-in sorted() function.

numbers = [42, 7, 19, 3] ordered = sorted(numbers)print(ordered)print(numbers) result = numbers.sort()print(result)print(numbers)

Try itChange line 7 to numbers = numbers.sort() and see what the last line prints.