A dict maps keys to values. Lookup by key takes about the same time however large the dict is. Dicts keep insertion order.

ages = {"ada": 36}
ages["alan"] = 41
print(ages.get("bob", 0))  # 0, no error
print("ada" in ages)       # True
for k, v in ages.items():
    print(k, v)

d[key] raises KeyError when the key is missing. d.get(key, default) does not. Keys must be hashable, so strings, numbers and tuples work and lists do not.

A set holds unique values with no useful order, and it supports membership tests and set algebra.

a = {1, 2, 3}
b = {2, 3, 4}
print(a & b)  # intersection
print(a | b)  # union
print(a - b)  # difference

Because a set's order is not guaranteed, sort it before showing or comparing it. Write set() for an empty set, since {} is an empty dict.

Write word_counts(text) returning a dict from lower-cased word to how many times it appears. Write common_items(a, b) returning a sorted list of the values present in both lists, without duplicates.

The explanation opens here once every check passes.