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✦ Python Intermediate & OOP Intermediate Python

L2 · Concept 01 · Advanced Python Techniques

Build collections
from one clear idea.

Comprehensions turn a familiar loop into a focused way to build a new list, set, or dictionary.

Learning goalUse list, set, and dictionary comprehensions to transform or filter collection data, and recognize when a regular for loop is clearer.
01 Learn02 Try03 Predict

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01 Learn

A comprehension builds a collection while it loops.

A regular for loop is often the clearest place to start. When the loop has one focused job—build a collection—a comprehension can express the same idea in one readable line.

numbers = [1, 2, 3, 4]

squares = []
for number in numbers:
    squares.append(number * number)

# The same focused transformation:
squares = [number * number for number in numbers]

Both versions visit each number and add its square to a new list. The comprehension keeps the result expression, the loop variable, and the source data together.

print(squares) # [1, 4, 9, 16]

The brackets choose the result type: [] makes a list, {} with one expression makes a set, and {key: value} makes a dictionary.

↳

Keep readability first: Use a regular loop when the work needs several steps, multiple side effects, or a long nested condition. A short comprehension should make code easier—not harder—to read.

Practical examples
names = ["ana", "marko", "jelena"]
uppercase_names = [name.upper() for name in names]

words = ["Python", "practice", "Python"]
initials = {word[0].lower() for word in words}

scores = {"Ana": 82, "Marko": 45, "Jelena": 91}
passed_scores = {name: score for name, score in scores.items() if score >= 60}

The first line transforms every name into a list. The second keeps unique initials in a set. The last one keeps only passing name-and-score pairs in a dictionary.

List comprehension — transform every item
prices = [100, 250, 80]
with_tax = [price * 1.2 for price in prices]

The expression before for is the value added for each price.

Set comprehension — keep unique results
cities = ["Novi Sad", "Niš", "Novi Sad"]
lengths = {len(city) for city in cities}

A set removes duplicate results. Do not use it when the displayed order matters.

Dictionary comprehension — create key-value pairs
words = ["python", "code"]
word_lengths = {word: len(word) for word in words}

Put the key and value before for. Repeated keys replace an earlier value, so choose keys carefully.

Read a comprehension from right to left

  1. 1. Start with the sourcefor name, score in scores.items() supplies one name-and-score pair at a time.
  2. 2. Choose the result expressionname, score * 1.2, or name: score describes what one iteration contributes.
  3. 3. Add an optional filterif score >= 60 keeps only iterations that meet the condition; it does not run an else branch.
  4. 4. Check the outer bracketsSquare brackets create a list, curly brackets create a set, and key: value inside curly brackets creates a dictionary.

Common mistakes

  • Choosing the wrong collectionUse a list for ordered results, a set for unique values, and a dictionary when each value needs a meaningful key.
  • Expecting a set orderSets are useful for uniqueness, not for a predictable display order.
  • Accidentally repeating a dictionary keyA later value for the same key replaces an earlier one.
  • Hiding too much work in one lineIf you need to explain the line several times, prefer a regular loop with named intermediate values.
02 Try

Python Playground

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Change the source data, result expression, or filter. Predict the new collection before you run the code.

comprehensions.pyPython
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03 Predict

What will this program print?

Trace the filter first. Only even numbers reach the expression before for.

numbers = [1, 2, 3, 4, 5]
doubled_evens = [number * 2 for number in numbers if number % 2 == 0]

print(doubled_evens)
Choose the output

Practice independently

Mini Challenge — Passing scores

Try this yourself before asking for a hint. You already know every building block you need.

Your goalBuild a list of passing student names, a dictionary of their scores, and—if you want an extra challenge—a set of unique scores.

passing_scores_challenge.py

Optional hints
  • Use scores.items() when you need a name and its score together.
  • The filter goes at the end: ... for name, score in scores.items() if score >= 60.
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