Nested dictionaries in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a nested dictionary in Python?

A nested dictionary is a dictionary where the values can also be dictionaries. It means you have dictionaries inside another dictionary, like boxes inside a bigger box.

beginner

How do you access a value inside a nested dictionary?

You use multiple square brackets, one for each dictionary level. For example, data['key1']['key2'] accesses the value inside the inner dictionary under 'key2'.

beginner

How can you add a new key-value pair inside a nested dictionary?

First access the inner dictionary, then assign a new key and value like data['outer_key']['new_key'] = value. This adds or updates the inner dictionary.

intermediate

What happens if you try to access a key that does not exist in a nested dictionary?

Python raises a KeyError because the key is missing. To avoid this, you can use methods like .get() or check if the key exists first.

intermediate

How can you loop through all keys and values in a nested dictionary?

Use a nested loop: the outer loop goes through the main dictionary, and the inner loop goes through each inner dictionary. This way you can access all keys and values at every level.

What is the correct way to access the value 42 in this nested dictionary?

{'a': {'b': 42}}

A data['a']['b']

B data['b']['a']

C data['a.b']

D data['a']['42']

You first access the outer key 'a', then the inner key 'b' to get 42.

How do you add a new key 'c' with value 100 inside the inner dictionary of {'a': {'b': 42}}?

A data['a'].append({'c': 100})

B data['c'] = 100

C data['a']['c'] = 100

D data['a', 'c'] = 100

You access the inner dictionary with data['a'] and assign the new key 'c' with value 100.

What error occurs if you try to access a missing key in a nested dictionary without checking?

A KeyError

B IndexError

C TypeError

D ValueError

Accessing a missing key in a dictionary raises a KeyError.

Which method safely gets a value from a nested dictionary without raising an error if the key is missing?

A .remove()

B .get()

C .pop()

D .append()

The .get() method returns None or a default value if the key is missing, avoiding KeyError.

How do you loop through all keys and values in a nested dictionary?

A Use recursion only

B Use a single for loop over the outer dictionary only

C Use a while loop with a counter

D Use nested for loops, one for each dictionary level

Nested for loops let you access keys and values at each level of the nested dictionary.

Explain what a nested dictionary is and how you access values inside it.

Think about dictionaries inside dictionaries and how you use multiple brackets.

Describe how to safely handle missing keys when working with nested dictionaries.

Consider what happens if a key is not found and how to avoid errors.

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