Dictionary Methods

Complete reference of all Python dictionary methods

Python Dictionary Methods

Python dictionaries are one of the most versatile and powerful data structures in the language. This page provides a comprehensive reference for all Python dictionary methods with examples, explanations, and time complexity analysis.

Creation and Basic Operations

Dictionary Creation

# Using curly braces
person = {'name': 'John', 'age': 30, 'city': 'New York'}

# Using dict() constructor
person = dict(name='John', age=30, city='New York')

# From a list of tuples
person = dict([('name', 'John'), ('age', 30), ('city', 'New York')])

# Dictionary comprehension
squares = {x: x**2 for x in range(6)}
# Result: {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

Time Complexity: O(n) where n is the number of key-value pairs

Accessing Values

person = {'name': 'John', 'age': 30, 'city': 'New York'}

# Using key
name = person['name']  # 'John'

# Using get() method (safer if key might not exist)
email = person.get('email')  # Returns None
email = person.get('email', 'Not available')  # Returns 'Not available'

Time Complexity: O(1) average case

Modification Methods

dict.update()

# Add or update multiple key-value pairs
person = {'name': 'John', 'age': 30}
person.update({'age': 31, 'city': 'New York'})
# Result: {'name': 'John', 'age': 31, 'city': 'New York'}

Time Complexity: O(n) where n is the length of the argument

dict.setdefault()

# Get value if key exists, otherwise insert key with a default value
person = {'name': 'John', 'age': 30}
city = person.setdefault('city', 'Unknown')
# person = {'name': 'John', 'age': 30, 'city': 'Unknown'}
# city = 'Unknown'

Time Complexity: O(1) average case

Modification Methods

dict.update()

# Add or update multiple key-value pairs
person = {'name': 'John', 'age': 30}
person.update({'age': 31, 'city': 'New York'})
# Result: {'name': 'John', 'age': 31, 'city': 'New York'}

Time Complexity: O(n) where n is the length of the argument

dict.setdefault()

# Get value if key exists, otherwise insert key with a default value
person = {'name': 'John', 'age': 30}
city = person.setdefault('city', 'Unknown')
# person = {'name': 'John', 'age': 30, 'city': 'Unknown'}
# city = 'Unknown'

Time Complexity: O(1) average case

dict.pop()

# Remove key and return its value
person = {'name': 'John', 'age': 30, 'city': 'New York'}
age = person.pop('age')
# person = {'name': 'John', 'city': 'New York'}
# age = 30

# With default value if key doesn't exist
email = person.pop('email', 'No email found')
# email = 'No email found'

Time Complexity: O(1) average case

dict.popitem()

# Remove and return the last inserted key-value pair
person = {'name': 'John', 'age': 30, 'city': 'New York'}
last_item = person.popitem()
# person = {'name': 'John', 'age': 30}
# last_item = ('city', 'New York')

Time Complexity: O(1)

dict.clear()

# Remove all items from the dictionary
person = {'name': 'John', 'age': 30, 'city': 'New York'}
person.clear()
# Result: {}

Time Complexity: O(1)

View Methods

dict.keys()

person = {'name': 'John', 'age': 30}
keys = person.keys()
# Result: dict_keys(['name', 'age'])

# Convert to list if needed
keys_list = list(person.keys())
# Result: ['name', 'age']

Time Complexity: O(1) for creating view, O(n) for conversion

dict.values()

person = {'name': 'John', 'age': 30}
values = person.values()
# Result: dict_values(['John', 30])

# Convert to list if needed
values_list = list(person.values())
# Result: ['John', 30]

Time Complexity: O(1) for creating view, O(n) for conversion

dict.items()

person = {'name': 'John', 'age': 30}
items = person.items()
# Result: dict_items([('name', 'John'), ('age', 30)])

# Iterate through key-value pairs
for key, value in person.items():
    print(key, value)
# Output:
# name John
# age 30

Time Complexity: O(1) for creating view, O(n) for iteration

Other Methods

dict.copy()

# Create a shallow copy
person = {'name': 'John', 'age': 30}
person_copy = person.copy()
# Both dictionaries have the same content
# but are different objects

Time Complexity: O(n)

dict.fromkeys()

# Create a new dictionary with keys from iterable
keys = ['name', 'age', 'city']
person = dict.fromkeys(keys, 'Unknown')
# Result: {'name': 'Unknown', 'age': 'Unknown', 'city': 'Unknown'}

Time Complexity: O(n)

Dictionary Membership

in Operator

person = {'name': 'John', 'age': 30, 'city': 'New York'}

# Check if key exists
if 'name' in person:
    print("Name found:", person['name'])

# Check if key doesn't exist
if 'email' not in person:
    print("Email not found")

Time Complexity: O(1) average case

Time Complexity Summary

OperationAverage CaseWorst CaseNotes
Access by key (d[key])O(1)O(n)Worst case happens with hash collisions
Set value (d[key] = value)O(1)O(n)Worst case happens with hash collisions
Delete key (del d[key])O(1)O(n)Worst case happens with hash collisions
get(key[, default])O(1)O(n)Safer than direct key access
update([other])O(n)O(n)n is the length of the other dictionary
keys(), values(), items()O(1)O(1)Returns view objects
copy()O(n)O(n)Creates a shallow copy
pop(key[, default])O(1)O(n)Removes item and returns value
popitem()O(1)O(1)Removes last inserted item
clear()O(1)O(1)Removes all items
fromkeys(seq[, value])O(n)O(n)n is the length of the sequence
in operatorO(1)O(n)Checks if key exists

Dictionary Best Practices

  • Use get() method instead of direct key access to avoid KeyError exceptions
  • Use dictionary comprehensions for creating dictionaries from existing data
  • When iterating, use items() to access both keys and values
  • Remember that dictionaries are ordered (Python 3.7+) but don't rely on this for older Python versions
  • Use dictionary unpacking (**dict) for combining dictionaries in Python 3.5+
  • For dictionaries with default values, consider using collections.defaultdict

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