Top 30 Python Dictionary Interview Questions

Python dictionaries are important data structures used to store key-value pairs. In this article, we will explore the Top 30 Python Dictionary interview questions to help you prepare for technical interviews.

Basic Python Dictionary Interview Questions

1. What is a Python Dictionary

A dictionary is an unordered, mutable collection of key-value pairs where each key must be unique and it map to a specific value.

Example:

dict = {'name': 'Alice', 'age': 25, 'city': 'New York'}

2. How can you access a value from a dictionary?

A dictionary's value can be accessed by using the key inside square brackets[ ].

Example:

a = {'name' :  'Alice' ,  'age' : '25'}
print(a['name']) 

3. What will happen if you access a non-existent key?

Whenever we try to access a key that doesn't exist, Python raises a KeyError. This error occurs because the key does not exist in the dictionary.

Example:

a = {'name': 'Alice'}
print(a['age'])  # Raises KeyError: 'age'

4. How can you avoid a KeyError while accessing a dictionary?

While accessing a key error we can use the get() method, which returns None (or a default value) if the key is not found instead of raising an error.

Example:

a = {'name': 'Alice'}

print(a.get('age'))  
print(a.get('age', 'Not Available'))  

5. How do you add or update a key-value pair in a dictionary?

We can add or update a key-value pair in a dictionary by directly assigning a value to a key. If the key already exists then its value is updated and if it doesn't exist, a new key-value pair is added.

Example:

a = {'name': 'Alice'}

# Adds new key-value pair
a['age'] = 25  
# Updates the value for the existing keys
a['age'] = 26   
print(a)  

6.How do you remove a key-value pair from a dictionary?

We can use del or the pop( ) method to remove a key-value pair. pop( ) also returns the value associated with the key.

Example:

a = {'name': 'Alice', 'age': 25}

# Removes 'age' key
del a['age']  
print(a)

# Removes 'name' key and returns its value
a.pop('name')  
print(a)  

7. How do you get all the keys, values, or items in a dictionary?

We can us the methods keys(), values(), or items() to get views of keys, values, or key-value pairs.

Example:

my_dict = {'name': 'Alice', 'age': 25}

#using keys()
print(my_dict.keys()) 
#using values()
print(my_dict.values())  
#using items()
print(my_dict.items()) 

9. How can you check if a key exists in a dictionary?

To check if a key exists in a dictionary, We can use the in keyword.

my_dict = {'name': 'Alice', 'age': 25}
if 'name' in my_dict:
    print("Key exists!") 

10. What is a nested dictionary?

A nested dictionary is a dictionary where the value of a key is another dictionary. This allows you to create more complex data structures where each key can store a dictionary, enabling hierarchical or multi-level data representation.

Example:

nested_dict = {'person': {'name': 'Alice', 'age': 25}, 'address': {'city': 'New York'}}

print(nested_dict)

11. What is the time complexity for accessing a value in a dictionary?

O(1) on average. Dictionary lookups are very efficient because they use a hash table to store key-value pairs.

12. What is the difference between a dictionary and a list in Python?

A dictionary is an unordered collection of key-value pairs, while a list is an ordered collection of elements. Dictionaries are better for fast lookups by key, while lists are useful for ordered collections.

Example:

my_list = [1, 2, 3]
my_dict = {'a': 1, 'b': 2, 'c': 3}

print(my_list)
print(my_dict)

13. How do you clear all elements from a dictionary?

To clear all elements from a dictionary we can use the clear( ) method to remove all key-value pairs. After calling clear(), the dictionary will no longer contain any data, and it will be an empty dictionary {}.

Example:

my_dict = {'name': 'Alice', 'age': 25}

#Using clear()
my_dict.clear()
print(my_dict)

14. How do you copy a dictionary?

The copy() method creates a shallow copy of the dictionary. This means it copies the dictionary structure itself but does not recursively copy nested objects (if any).

Example:

original_dict = {'name': 'Alice', 'age': 25}

#Using copy() 
copy_dict = original_dict.copy()
print(copy_dict) 

15. How do you iterate over a dictionary?

We can iterate over a dictionary using a for loop. Use items() to iterate over both keys and values.

Example:

my_dict = {'name': 'Alice', 'age': 25}
for key, value in my_dict.items():
    print(f"{key}: {value}")

Intermediate Python Dictionary Interview Questions

16. What is the difference between pop() and popitem()?

• pop() removes a key-value pair by the key and returns its value.
• popitem() removes and returns a random key-value pair, typically used for arbitary removal.

17. How can you merge two dictionaries?

We can merge two dictionaries using update() or by using dictionary unpacking (**).

Example:

dict1 = {'name': 'Alice'}
dict2 = {'age': 25}
dict1.update(dict2)  # Merges dict2 into dict1
print(dict1)  

# Using unpacking
merged_dict = {**dict1, **dict2}
print(merged_dict)

18. What is the setdefault() method in dictionaries?

The setdefault() method returns the value of a key if it exists, otherwise, it inserts the key with a specified default value.

Example:

my_dict = {'name': 'Alice'}

 # Adds 'age' with default value 25
value = my_dict.setdefault('age', 25) 
print(my_dict)  

19. What is the purpose of the fromkeys() method?

The fromkeys() method creates a new dictionary from a sequence of keys, assigning them the same initial value.

Example:

keys = ['a', 'b', 'c']

#Using fromkeys 
my_dict = dict.fromkeys(keys, 0)
print(my_dict)

20. What is enumerate function inside a dictionary?

Enumerate function is used to get position index and corresponding index at the same time.

Example:

dict1 = {'name' : 'Alice', 'age' : 25}
for i in enumerate(dict1):
    print(i)

21. How can you convert a list of tuples into a dictionary?

We can use the dict() constructor to convert a list of tuples into a dictionary. Each tuple should have exactly two elements: the first element will be used as the key, and the second element will be used as the value.

tuple_list = [('a', 1), ('b', 2)]

my_dict = dict(tuple_list)
print(my_dict)  

22. What is a defaultdict?

A defaultdict is a subclass of dict that provides a default value if a key is not found. It can be initialized with a factory function.

Example:

from collections import defaultdict
my_dict = defaultdict(int)  # Default value is 0 for missing keys
my_dict['a'] += 1
print(my_dict)  

23. What is the items() method used for?

The items() method returns a view object that displays a list of the dictionary’s key-value pairs as tuples.

Example:

my_dict = {'name': 'Alice', 'age': 25}

#using items
print(my_dict.items())  

24. Can dictionary keys be mutable types like lists?

No, dictionary keys must be immutable types (e.g., strings, numbers, tuples). Mutable types like lists or dictionaries cannot be used as dictionary keys.

25. How do you check the size of a dictionary?

We can use the len() function to get the number of key-value pairs in a dictionary. This function returns the number of items (key-value pairs) in the dictionary.

my_dict = {'name': 'Alice', 'age': 25}

# Checking the size of the dictionary
print(len(my_dict))  

Python Dictionary Interview Questions for Experienced

26. How can you access values in a nested dictionary?

To access values in a nested dictionary, you can use multiple keys, one for each level of the dictionary. We access the value at the first level by specifying its key, and then we can continue accessing deeper levels using additional keys.

Example:

nested_dict = {'person': {'name': 'Alice', 'age': 25}, 'address': {'city': 'New York'}}
print(nested_dict['person']['name']) 

27. How do you handle exceptions when accessing keys in large dictionaries with nested structures?

We can use try-except blocks or the get() method to prevent errors when accessing non-existent keys.

Example:

nested_dict = {'person': {'name': 'Alice', 'age': 25}, 'address': {'city': 'New York'}}
try:
    value = nested_dict['person']['salary']
except KeyError:
    print("Key not found!")

28. What is the difference between shallow copy and deep copy of a dictionary?

There is a basic difference between a Shallow and a deep copy is - A shallow copy copies the dictionary structure but not the nested objects, while a deep copy recursively copies both the dictionary structure and all nested objects, creating independent copies.

29. How do you efficiently update a dictionary using another dictionary?

The update() method updates the dictionary with the keys and values from another dictionary, modifying the original dictionary. Dictionary unpacking (**) creates a new dictionary by merging two dictionaries, with the latter dictionary's values overwriting the former's in case of key conflicts.

Example:

dict1 = {'name': 'Alice'}
dict2 = {'age': 25}
dict1.update(dict2)
print(dict1)  

30. Explain dictionary comprehensions and give an example.

Dictionary comprehensions allows us to create dictionaries from iterable objects in a single line. It follows the pattern {key: value for key, value in iterable}.

Example:

my_dict = {x: x**2 for x in range(5)}
print(my_dict)