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Dictionaries – Iteration

Iterating a dictionary with a for loop visits each key in turn

Part of: Python Coding — Module 39, 9-10 Digital Technologies: Data structures in Python — Module 8, WA Computer Science ATAR Year 11 — Control Structures and Data Types — Module 60

Understanding Dictionaries – Iteration in Python

What it does: .items() provides (key, value) tuples. k, v unpacks each pair. 'in' checks if a key exists in the dict — prevents KeyError. Iterating a dict directly gives you the keys — same as for key in d.keys():. You must call .values() explicitly — for v in d: gives keys, not values. .items() returns (key, value) pairs — then unpack them into k and v. Looping over d directly yields the keys — 'a' and 'b', not the values. Python uses the in operator — dicts don't have a .has() method. Changing a dict's size mid-loop raises RuntimeError — loop a copy instead.

Common mistakes: Looping over d directly gives only keys. Use .items() for key-value pairs. for val in d: gives KEYS, not values. Use .values() for values.

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This module's key terms are also in a printable crossword (Modules 31–40) and word search (Modules 21–40).

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Learning

Duo

for key in d: what does key get?

Each key, one at a timeEach value

Matches

❌ d = {'a':1} for k, v in d: print(k, v)

↓

Error: Must use d.items()

Applying

Meanings

Term

for key in d:

Meaning

Iterate through all keys in the dictionary.

Practice

d = {'a': 1, 'b': 2, 'c': 3}

Testing

Gaps

for key d:

Check Answer

Spots

1d = {'a': 1, 'b': 2}

2for k, v in d:

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Quick Questions

1. for key in d: what does key get?

A. Each key, one at a time ✓

B. Each value

Iterating a dict directly gives you the keys — same as for key in d.keys():

2. How do you loop through values?

A. for v in d.values(): ✓

B. for v in d:

You must call .values() explicitly — for v in d: gives keys, not values

3. How do you loop through keys AND values?

A. for k, v in d.items(): ✓

B. for k, v in d:

.items() returns (key, value) pairs — then unpack them into k and v

4. d = {'a':1, 'b':2}\nfor k in d:\n print(k)\nWhat prints?

A. 1 then 2

B. a then b ✓

Looping over d directly yields the keys — 'a' and 'b', not the values

5. d = {'x':10}\nfor k, v in d.items():\n print(k, v)\nWhat prints?

A. x

B. x 10 ✓

.items() unpacks each pair — k gets 'x' and v gets 10

Match the Pairs

d = {'a':1} for k, v in d: print(k, v) → Error: Must use d.items()

d = {'x':1} d['x'] += 1 del d['x'] print(d['x']) → Error: Key deleted, KeyError

for k, v in d.items(): print(k, v) → Valid: Unpack key and value

if 'name' in d: print(d['name']) → Ok: Check key before accessing

d = {'a':1,'b':2} for k in d: print(k) → Output: a b

d = {'x':10} for k,v in d.items(): print(k,v) → Output: x 10

Key Term

for key in d:

Iterate through all keys in the dictionary.~~for v in d.values():

Fill in the Blanks

1. for key in d: print(key)

2. for val in d.values(): print(val)

3. for k, v in d.items(): print(k, v)

4. if 'name' in d: print('Found')

5. for key in d: print(d[key])

Spot the Error

Question 1

A d = {'a': 1, 'b': 2}

B for k, v in d:

C print(k, v)

D print('Done')

Line B — Looping over d directly gives only keys. Use .items() for key-value pairs.

Question 2

A d = {'a': 1, 'b': 2}

B for val in d:

C print(val)

D # expects values

Line B — for val in d: gives KEYS, not values. Use .values() for values.

Question 3

A d = {'a': 1, 'b': 2}

B for k, v in d.keys():

C print(k, v)

D print('Done')

Line B — .keys() only gives keys. Use .items() to get both key and value.

Question 4

A d = {'a': 1, 'b': 2}

B for k in d.items():

C print(k)

D print('Done')

Line B — Without unpacking, k is a tuple like ('a', 1). Use k, v to unpack.

Question 5

A found = 'Alice' in {'name': 'Alice', 'age': 12}

B d = {'name': 'Alice', 'age': 12}

C print('Found:', found)

D # expects True

Line A — 'in' checks KEYS by default. To check values, use d.values().

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