Python / Core Python Fundamentals Interview Questions
How do Python dictionaries work and what are the most important operations?
A dictionary is Python's hash-map: an unordered (insertion-ordered since Python 3.7) collection of key-value pairs. Keys must be hashable (strings, numbers, tuples of hashable items); values can be anything. Lookup, insertion, and deletion are O(1) average-case.
user = { 'name': 'Alice', 'age': 30, 'active': True } # Access â raises KeyError if key missing print(user['name']) # Alice # Safe access â returns default if key missing print(user.get('email', 'N/A')) # N/A # Add / update user['email'] = 'alice@example.com' user.update({'age': 31, 'city': 'NYC'}) # Delete del user['active'] role = user.pop('city', None) # removes and returns; default avoids KeyError # Iterating for key, value in user.items(): print(f'{key}: {value}') # Keys and values as views print(list(user.keys())) # ['name', 'age', 'email'] print(list(user.values())) # ['Alice', 31, 'alice@example.com']
Dict comprehension builds dictionaries from iterables in one line:
squares = {x: x**2 for x in range(1, 6)} # {1:1, 2:4, 3:9, 4:16, 5:25}
Checking membership tests keys only: 'name' in user is O(1). To check values you must iterate, which is O(n). For counting occurrences, collections.Counter is a dict subclass that auto-initialises missing keys to zero, making frequency analysis much cleaner.
What does dict.get('missing_key') return when the key does not exist and no default is provided?
Since which Python version are dictionaries guaranteed to maintain insertion order?
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