Dictionary iteration in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of iterating dictionary keys

What is the output of this code?

my_dict = {'a': 1, 'b': 2, 'c': 3}
result = []
for key in my_dict:
    result.append(key)
print(result)

Python

my_dict = {'a': 1, 'b': 2, 'c': 3}
result = []
for key in my_dict:
    result.append(key)
print(result)

A ['a', 1, 'b', 2, 'c', 3]

B [1, 2, 3]

C ['a', 'b', 'c']

D []

💡 Hint

Iterating a dictionary by default goes over its keys.

When you loop over a dictionary, you get its keys one by one. So appending keys to a list results in ['a', 'b', 'c'].

❓ Predict Output

intermediate

Output of iterating dictionary items

What is the output of this code?

my_dict = {'x': 10, 'y': 20}
result = []
for k, v in my_dict.items():
    result.append((k, v))
print(result)

Python

my_dict = {'x': 10, 'y': 20}
result = []
for k, v in my_dict.items():
    result.append((k, v))
print(result)

A [('x', 10), ('y', 20)]

B ['x', 'y']

C [10, 20]

D [('x', 'y'), (10, 20)]

💡 Hint

items() returns key-value pairs as tuples.

The items() method returns pairs of (key, value). Appending these pairs creates a list of tuples.

❓ Predict Output

advanced

Output of modifying dictionary during iteration

What happens when you run this code?

my_dict = {'a': 1, 'b': 2}
for key in my_dict:
    my_dict['c'] = 3
print(my_dict)

Python

my_dict = {'a': 1, 'b': 2}
for key in my_dict:
    my_dict['c'] = 3
print(my_dict)

A {'a': 1, 'b': 2, 'c': 3}

B RuntimeError: dictionary changed size during iteration

C {'a': 1, 'b': 2}

D SyntaxError

💡 Hint

Changing a dictionary while looping over it causes an error.

Python does not allow changing the size of a dictionary during iteration. Adding a new key causes a RuntimeError.

🧠 Conceptual

advanced

Number of items after filtering dictionary keys

Given this code, how many items will the resulting dictionary have?

original = {'a': 1, 'b': 2, 'c': 3, 'd': 4}
filtered = {k: v for k, v in original.items() if v % 2 == 0}
print(len(filtered))

Python

original = {'a': 1, 'b': 2, 'c': 3, 'd': 4}
filtered = {k: v for k, v in original.items() if v % 2 == 0}
print(len(filtered))

A 1

B 3

C 4

D 2

💡 Hint

Count how many values are even numbers.

Only values 2 and 4 are even, so filtered dictionary has 2 items.

🔧 Debug

expert

Identify the error in dictionary iteration code

What error does this code raise?

data = {'x': 5, 'y': 10}
for key, value in data:
    print(key, value)

Python

data = {'x': 5, 'y': 10}
for key, value in data:
    print(key, value)

A ValueError: not enough values to unpack (expected 2, got 1)

B KeyError

C SyntaxError

D No error, prints keys and values

💡 Hint

Iterating a dictionary yields keys, which are strings here.

Loop tries to unpack each key (a string) into two variables, causing ValueError: not enough values to unpack (expected 2, got 1).