You already met list comprehensions in fundamentals — the [x for x in xs] shorthand. This lesson goes deeper and adds two siblings: dicts and sets. Same shape, different brackets.
What you'll learn
- Build dicts and sets in one line, not just lists
- Combine filtering and transformation in a single expression
- Know when a comprehension stops being clearer than a regular loop
Instructions
You're given a list of student tuples — each is a name and a numeric grade out of 100. Build three things from it and print each on its own line:
- A list of the names of students who passed (grade ≥ 60). Print it as
passing students: <list>. - A dict mapping every student's name to their grade. Print it as
grades by student: <dict>. - A set of the unique tens-digit of every grade (e.g., 87 → 8, 54 → 5). Print it as
unique grade tens: <sorted list>.
Sets don't have a stable print order — the same set can render {5, 8, 9} one run and {8, 9, 5} the next. To make the output predictable, the starter calls sorted(grade_tens) for the print line. Build the value as a set (set comprehension is the lesson); just print it sorted.
The students list is given for you in the starter. Your output must match exactly:
passing students: ['Alice', 'Cara']
grades by student: {'Alice': 87, 'Bob': 54, 'Cara': 91}
unique grade tens: [5, 8, 9]Key concepts
List comprehension with a filter
You saw the basics in fundamentals. Worth a recap because the shape is the same for dicts and sets:
nums = [1, 2, 3, 4, 5]
evens = [n for n in nums if n % 2 == 0]
# evens is [2, 4]Read it left-to-right: build a list […] with n for each n in nums where n is even. The if clause is optional.
Dict comprehension
Same idea, but you write a key: value pair instead of a single expression:
words = ["hi", "hello"]
lengths = {w: len(w) for w in words}
# lengths is {'hi': 2, 'hello': 5}Notice the curly braces and the colon — that's how Python knows it's a dict, not a set.
Set comprehension
Curly braces, no colon — Python infers a set:
nums = [1, 2, 2, 3, 3, 3]
unique = {n for n in nums}
# unique is {1, 2, 3}Sets drop duplicates automatically. Useful when you want to know "what distinct values appear here?"
When to NOT use a comprehension
Comprehensions are a shortcut, not a hammer. If the body needs more than one short expression — multiple statements, complex branching, side effects like printing — a regular for loop is clearer:
# Good: one expression, easy to read
squares = [n * n for n in range(10)]
# Bad: cramming logic in — use a loop instead
results = [process(n) if n > 0 else fallback(n) for n in items if n is not None]A rough rule: if you can't read it left-to-right and immediately see what it builds, it's too much.
Hints
(These are surfaced by the tutor on request — they don't auto-reveal.)
- You need three different collection types. Each has its own bracket style:
[...],{...}, and{k: v for ...}. - For the passing-students list, filter the students with
if grade >= 60. Each tuple unpacks into two names:for name, grade in students. - The tens digit of a number is
grade // 10(integer division — drops the units digit).