Python for JS Devs · Fundamentals

Collections: list, dict, tuple, set

Your array and object have four Python cousins. Knowing which to reach for is half of writing idiomatic Python.

JS gives you Array, Object, Map, Set. Python gives you list, dict, tuple, set. The mapping is close but not 1:1 — and the mismatches are where the value is.

list — the workhorse array

xs = [1, 2, 3];
xs.push(4);
xs.length;
xs.includes(2);
xs.slice(1, 3);

xs = [1, 2, 3]
xs.append(4)     # not push
len(xs)          # len() function, not .length
2 in xs          # `in`, not .includes
xs[1:3]         # slicing syntax

Slicing is a Python superpower: xs[start:stop:step], and negatives count from the end. xs[-1] is the last item; xs[::-1] reverses.

dict — not the same as a JS object

A JS object doubles as "struct" and "map". A Python dict is purely a map — and you access it with brackets, not dots.

u = { name: "kd", age: 30 };
u.name;
u.email ?? "none";
"name" in u;
Object.keys(u);

u = {"name": "kd", "age": 30}
u["name"]              # KeyError if missing!
u.get("email", "none")  # safe default (?? equivalent)
"name" in u             # checks keys
u.keys(), u.values(), u.items()

tuple — the one with no JS equivalent

An immutable, fixed-shape sequence. You'll meet it everywhere: returning multiple values, dict keys, fixed records. Unpacking is the killer feature (like array destructuring, but pervasive).

point = (10, 20)
x, y = point            # unpacking, like [x, y] = arr

def min_max(xs):
    return min(xs), max(xs)   # returns a tuple

low, high = min_max([3, 1, 9])   # unpack the result

set — dedup & membership

seen = {1, 2, 2, 3}    # → {1, 2, 3}
3 in seen                # O(1) membership
a | b                    # union     a & b → intersection
set([1,1,2])            # dedup a list

Check yourself