How to Iterate Over a Python Dictionary — GCSE Computer Science

What happens when you loop over a dictionary directly?

If you write a for loop with just the dictionary name, Python iterates over the keys by default:

scores = {"Alice": 88, "Bob": 72, "Chloe": 95}

for student in scores:
    print(student)
# Output:
# Alice
# Bob
# Chloe

This is identical to writing for student in scores.keys(). The values are not directly accessible inside the loop — you must use scores[student] to look them up.

How do you iterate over keys, values, and key-value pairs?

Follow these steps to choose the correct iteration method for a given task:

1. Use .keys() to loop through dictionary keys. This is the default, but writing .keys() makes your intent explicit and readable. Use it when you need only the key and will look up values manually.
2. Use .values() to loop through dictionary values only. Use this when you need every value for a calculation but do not need the associated key — for example, summing all scores.
3. Use .items() to loop through both keys and values together. Python unpacks each entry into two variables simultaneously. This is the most commonly used method for tasks that need both the key and the value in the same iteration.

scores = {"Alice": 88, "Bob": 72, "Chloe": 95}

# Step 1: iterate over keys
for name in scores.keys():
    print(name)

# Step 2: iterate over values
total = 0
for score in scores.values():
    total += score
print("Total:", total)          # Total: 255

# Step 3: iterate over key-value pairs
for name, score in scores.items():
    print(f"{name}: {score}")
# Output:
# Alice: 88
# Bob: 72
# Chloe: 95

How do you find the highest or lowest value in a dictionary?

A common GCSE task is to find the key with the maximum or minimum value. You can combine .items() with max() or use a loop:

scores = {"Alice": 88, "Bob": 72, "Chloe": 95}

# Using max() with a key function
top_student = max(scores, key=scores.get)
print(top_student)              # Chloe

# Using a loop (easier to understand at GCSE)
best_name = ""
best_score = -1
for name, score in scores.items():
    if score > best_score:
        best_score = score
        best_name = name
print(best_name, best_score)    # Chloe 95

How do you count occurrences with a dictionary?

Dictionaries are ideal for tallying frequencies — building a count of how many times each item appears in a list:

words = ["cat", "dog", "cat", "fish", "dog", "cat"]
counts = {}

for word in words:
    if word in counts:
        counts[word] += 1
    else:
        counts[word] = 1

for word, count in counts.items():
    print(f"{word}: {count}")
# cat: 3
# dog: 2
# fish: 1

This pattern — checking membership with in then updating or inserting — appears frequently in GCSE programming tasks.

What is a dictionary comprehension?

A dictionary comprehension creates a new dictionary in a single expression, similar to a list comprehension. The syntax is {key: value for item in iterable}:

scores = {"Alice": 88, "Bob": 72, "Chloe": 95}

# Create a new dict with grades A/B/C
grades = {
    name: "A" if score >= 90 else "B" if score >= 75 else "C"
    for name, score in scores.items()
}
print(grades)
# {'Alice': 'B', 'Bob': 'C', 'Chloe': 'A'}

Frequently asked questions

In what order does Python iterate over a dictionary?

From Python 3.7 onwards, dictionaries maintain insertion order — meaning a for loop visits entries in the order they were added. Earlier versions of Python 3 (and Python 2) made no guarantee about order. At GCSE you can rely on insertion order when using Python 3, but it is good practice to mention this assumption explicitly.

Can you modify a dictionary while iterating over it?

No — modifying a dictionary's size (adding or deleting keys) during a for loop raises a RuntimeError: dictionary changed size during iteration. If you need to remove entries based on a condition, collect the keys to delete in a separate list first, then loop over that list to delete them after the main iteration.

What is the difference between dict.items() and dict.keys()?

.keys() returns a view of just the keys; .values() returns just the values; .items() returns each key-value pair as a tuple. The for name, score in scores.items() syntax uses tuple unpacking — Python automatically assigns the first element of each tuple to name and the second to score.

Why is iterating with .items() better than using square-bracket lookup?

Writing for name in scores: print(scores[name]) works but performs a dictionary lookup (scores[name]) on every iteration. Writing for name, score in scores.items() gives you both values directly without an extra lookup. For small dictionaries the difference is negligible, but .items() is considered cleaner, more Pythonic, and is what most GCSE mark schemes expect.

---

For Socratic GCSE tutoring and free Computer Science flashcards, see aitutors.me.