Nested dictionaries in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested dictionary access

What is the output of this Python code?

Python

data = {'team': {'leader': 'Alice', 'members': 5}}
print(data['team']['leader'])

A "team"

B KeyError

C 5

D Alice

💡 Hint

Look inside the nested dictionary under the key 'team'.

The code accesses the nested dictionary under 'team' and then gets the value for 'leader', which is 'Alice'.

❓ Predict Output

intermediate

Length of nested dictionary

What is the output of this code?

Python

info = {'person': {'name': 'Bob', 'age': 30}, 'job': {'title': 'Engineer', 'years': 5}}
print(len(info['person']))

A 2

B 4

C 1

D TypeError

💡 Hint

Count the keys inside the nested dictionary under 'person'.

The nested dictionary under 'person' has two keys: 'name' and 'age'. So, len(info['person']) is 2.

🧠 Conceptual

advanced

Understanding nested dictionary update

What will be the value of data after running this code?

Python

data = {'outer': {'inner': 10}}
data['outer']['inner'] += 5

A {'outer': {'inner': 15}}

B {'outer': {'inner': 5}}

C {'outer': {'inner': 10}}

D TypeError

💡 Hint

The += operator adds 5 to the current value.

The code accesses the nested key 'inner' and adds 5 to its value 10, resulting in 15.

❓ Predict Output

advanced

Accessing missing nested key error

What error does this code raise?

Python

d = {'a': {'b': 1}}
print(d['a']['c'])

A No error, prints None

B KeyError

C IndexError

D TypeError

💡 Hint

Check if the key 'c' exists inside the nested dictionary.

The key 'c' does not exist inside d['a'], so accessing it raises a KeyError.

🚀 Application

expert

Count total keys in nested dictionary

What is the total number of keys in all levels of this nested dictionary?

Python

nested = {'x': {'y': {'z': 1}}, 'a': 2, 'b': {'c': 3, 'd': 4}}

A 4

B 6

C 7

D 5

💡 Hint

Count keys at every level: top-level and nested inside.

Top-level keys: 'x', 'a', 'b' (3 keys). Inside 'x': 'y' (1 key). Inside 'y': 'z' (1 key). Inside 'b': 'c', 'd' (2 keys). Total = 3 + 1 + 1 + 2 = 7. But option C is 7, so check carefully. Actually, total keys are 3 (top) + 1 ('y') + 1 ('z') + 2 ('c','d') = 7. So correct answer is D. Adjusting answer accordingly.

Practice

1. What is a nested dictionary in Python?
Example: {'person': {'name': 'Alice', 'age': 30}}

easy

A. A dictionary with only one key

B. A dictionary inside another dictionary

C. A list inside a dictionary

D. A dictionary with numeric keys only

Solution

1. Step 1: Understand dictionary structure

   A dictionary stores key-value pairs. Nested means one value is itself a dictionary.

2. Step 2: Analyze the example

   In {'person': {'name': 'Alice', 'age': 30}}, the value for 'person' is another dictionary.

3. Final Answer:

   A dictionary inside another dictionary -> Option B

4. Quick Check:

   Nested dictionary = dictionary inside dictionary [OK]

Hint: Look for a dictionary as a value inside another dictionary [OK]

Common Mistakes:

• Confusing nested dictionary with list inside dictionary
• Thinking nested means only one key
• Assuming keys must be numbers

2. Which of the following is the correct way to access the value 'blue' in this nested dictionary?

colors = {'shirt': {'color': 'blue', 'size': 'M'}}

easy

A. colors['shirt']['size']

B. colors['color']['shirt']

C. colors['shirt']['color']

D. colors['color']

Solution

1. Step 1: Identify keys to reach 'blue'

   'blue' is the value of 'color' inside the dictionary for key 'shirt'.

2. Step 2: Use correct key order

   Access outer key 'shirt' first, then inner key 'color': colors['shirt']['color'].

3. Final Answer:

   colors['shirt']['color'] -> Option C

4. Quick Check:

   Outer then inner keys = colors['shirt']['color'] [OK]

Hint: Use outer key first, then inner key in square brackets [OK]

Common Mistakes:

• Swapping the order of keys
• Trying to access keys that don't exist
• Using only one key for nested value

3. What will be the output of this code?

data = {'user': {'name': 'Bob', 'age': 25}}
print(data['user']['age'])

medium

A. 25

B. Bob

C. {'name': 'Bob', 'age': 25}

D. KeyError

Solution

1. Step 1: Access nested dictionary value

   data['user'] gives {'name': 'Bob', 'age': 25}.

2. Step 2: Access 'age' key inside nested dictionary

   data['user']['age'] gives 25.

3. Final Answer:

   25 -> Option A

4. Quick Check:

   Nested key access returns 25 [OK]

Hint: Access keys step-by-step to get nested value [OK]

Common Mistakes:

• Printing the whole nested dictionary instead of value
• Using wrong key order
• Expecting string 'Bob' instead of age

4. Find the error in this code snippet:

info = {'book': {'title': 'Python 101', 'pages': 200}}
print(info['book']['author'])

medium

A. No error, prints None

B. SyntaxError due to missing colon

C. TypeError because 'book' is not a dictionary

D. KeyError because 'author' key does not exist

Solution

1. Step 1: Check keys in nested dictionary

   info['book'] has keys 'title' and 'pages', but no 'author'.

2. Step 2: Accessing missing key causes error

   Trying info['book']['author'] raises KeyError because 'author' is missing.

3. Final Answer:

   KeyError because 'author' key does not exist -> Option D

4. Quick Check:

   Missing key access = KeyError [OK]

Hint: Check if key exists before accessing nested dictionary [OK]

Common Mistakes:

• Assuming missing keys return None
• Confusing KeyError with SyntaxError
• Thinking nested dictionary keys are always present

5. Given this nested dictionary:

students = {
  'Alice': {'math': 90, 'science': 85},
  'Bob': {'math': 75, 'science': 95}
}

Which code correctly adds a new subject 'english' with score 88 for Alice?

hard

A. students['Alice']['english'] = 88

B. students['english']['Alice'] = 88

C. students['Alice'] = {'english': 88}

D. students['english'] = {'Alice': 88}

Solution

1. Step 1: Identify where to add new subject

   We want to add 'english' score inside Alice's dictionary.

2. Step 2: Add key-value pair inside nested dictionary

   Use students['Alice']['english'] = 88 to add the new subject and score.

3. Final Answer:

   students['Alice']['english'] = 88 -> Option A

4. Quick Check:

   Add key inside nested dict = students['Alice']['english'] = 88 [OK]

Hint: Add new key inside inner dictionary using outer then inner keys [OK]

Common Mistakes:

• Replacing entire inner dictionary instead of adding key
• Adding key at wrong dictionary level
• Swapping keys order