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Python Dictionaries

Introduction

A Python dictionary (dict) is a mutable, unordered (historically) collection of key-value pairs.

(Note: As of Python 3.7, dictionaries maintain insertion order, but you should rarely rely on this property in algorithm interviews unless specifically using collections.OrderedDict).

Creating Dictionaries

You can create dictionaries using curly braces {} or the dict() constructor.

# Empty dictionary
empty_dict = {}

# Initialized dictionary
person = {
    "name": "Alice",
    "age": 25,
    "city": "New York"
}

Accessing and Updating Values

You access values using square bracket notation.

print(person["name"]) # "Alice"

If you access a key that does not exist, Python raises a KeyError. 

# KeyError: 'job'
# print(person["job"])

To update a value or add a new key-value pair, assign a value to the key.

# Update existing key
person["age"] = 26

# Add new key
person["job"] = "Engineer"

The in Operator (Crucial for Interviews)

To check if a key exists in a dictionary, use the in operator. This is an \(O(1)\) operation and is the most common way to prevent KeyErrors.

if "job" in person:
    print(person["job"])
else:
    print("Job not found")

Common Interview Mistake: Checking Values

The in operator only checks keys, not values.

# This is False, because "Alice" is a value, not a key
print("Alice" in person)
If you need to check if a value exists, you must use val in person.values(), which takes \(O(N)\) time.

Deleting Elements

You can remove elements using the del keyword or the pop() method.

# Using del (raises KeyError if key doesn't exist)
del person["city"]

# Using pop (returns the value, allows a default to avoid KeyError)
job = person.pop("job", "Unemployed")

Time and Space Complexity

  • Time Complexity:
  • Insert: \(O(1)\) average
  • Lookup: \(O(1)\) average
  • Delete: \(O(1)\) average
  • key in dict: \(O(1)\) average
  • Space Complexity: \(O(N)\) to store \(N\) key-value pairs.

Summary

  • Use {} to create a dictionary.
  • Keys must be immutable (strings, integers, tuples).
  • Use key in dict to check for key existence in \(O(1)\) time.
  • del dict[key] or dict.pop(key) removes elements.