Dictionaries — Iteration & Methods

Keys, values, and items

Dictionaries store pairs, so you often loop through keys, values, or both.

python
scores = {"Ali": 91, "Mona": 88}

Looping over keys (default)

By default, a for loop gives you keys.

python
for name in scores:
    print(name)

Looping over values

Use values() when you only need the values.

python
for score in scores.values():
    print(score)

Looping over key and value

Use items() to get both key and value at the same time.

python
for name, score in scores.items():
    print(name, score)

Building a new dictionary

Create a new dict instead of modifying the one you are looping.

python
curved = {}
for name, score in scores.items():
    curved[name] = score + 5

Safer lookups with get()

get() avoids KeyError when a key may be missing.

python
print(scores.get("Sam", 0))  # 0 if missing

setdefault() for grouping

setdefault() creates a list for a key if it is missing.

python
errors = {}
errors.setdefault("E101", []).append("missing file")

Counting frequencies (classic pattern)

This is a common real-world use of dictionaries.

python
text = "banana"
counts = {}
for ch in text:
    counts[ch] = counts.get(ch, 0) + 1

Sorting keys or items

If you need sorted output, wrap keys or items with sorted().

python
for name in sorted(scores.keys()):
    print(name)

Do not change size while iterating

Adding or removing keys during a loop can cause errors or skipped items. Build a new dict instead.

Common mistakes to avoid

• Using keys() when you need both key and value.
• Forgetting that get() can return None if no default is given.
• Modifying a dict while looping over it.

What you should understand after this lesson

• How to loop over keys, values, and items.
• How to build new dicts from old ones.
• How to use get() and setdefault() safely.
• Why changing size during iteration is risky.