Dictionaries & Sets

What does this print? ``` d = {"a": 1, "a": 2} print(d) ```

Why does this raise an error? ``` d = {[1, 2]: "x"} ```

Can a tuple be a dict key but a list cannot? Why the difference?

What does this print? ``` d = {1: "x"} print(d[1.0]) ```

What does this print? ``` d = {"a": 1, "b": 2} print("a" in d) print(1 in d) ```

What does this loop print? ``` d = {"a": 1, "b": 2} for x in d: print(x) ```

Are dict keys iterated in a guaranteed order? Since when?

What's the difference between d[k], d.get(k), and d.setdefault(k, x) for a missing key?

What does this print? ``` d = {"k1": "v1"} print(d.get("k2", 99)) print(d) ```

What does this print? ``` a = {"x": 1, "y": 2} b = {"y": 9, "z": 3} print({**a, **b}) ```

Which Python version added the | and |= merge operators for dicts, and what's the pre-3.9 way?

What does this print? ``` d = {"a": 1} view = d.keys() d["b"] = 2 print(list(view)) ```

What happens? ``` d = {"a": 1, "b": 2} for k in d: if k == "a": del d[k] ```

What does this print, and why is it a classic bug? ``` def f(k, cache={}): cache[k] = k * k return cache f(2) print(f(3)) ```

What does this print? ``` a = {"x": 1, "y": 2} b = {"y": 2, "x": 1} print(a == b) ```

What does this print? ``` d = {"a": 1, "b": 2} v = d.pop("a") print(v, d) ```

What does this comprehension produce? ``` {k: len(k) for k in ["hi", "bye"]} ```

What does this print? ``` s = {1, 2, 2, 3, 3, 3} print(len(s)) ```

What's the difference between set add/update and discard/remove?

What does this print? ``` s = {1, 2, 3, 4, 5} t = {4, 5, 6, 7} print(s & t, s - t, s ^ t) ```

What does this print? ``` m = {1, 2, 3} n = {1, 2, 3} print(m <= n, m < n) ```

Why use frozenset, and what does the difference let you do?

After d.clear(), what is d — and how is that different from del d?

What does dict.fromkeys(["a", "b", "c"], 0) build, and what's the trap when the default value is mutable?

Do sets keep insertion order like dicts? Why might {'A', 'B', 'C'} print in a different order across separate runs?

What does this print? print({1, 2, *[3, 4]})

Sets support |=, &=, -=, and ^=. What actually happens when you apply |= to a frozenset?

Which set operation is available as a method but has no operator equivalent, and what does it test?

What does this print? print({1, 2, 3}.union("AB")) print({1, 2, 3} | "AB")

Besides a {...} literal, what two call forms does the dict() constructor accept — and what restricts the keyword form?

What does this print? d1 = {"k": [1, 2, 3]} d2 = d1.copy() d2["k"].append(4) print(d1["k"])

What does popitem() return and remove from a dict, and what happens when the dict is empty?

What type is created by {} versus set()?