Dictionaries & Sets in Python

Dictionaries

Dictionaries store key-value pairs and are mutable.

Creating Dictionaries

dct = {'a': 1, 'b': 2, 'c': 3}
empty_dct = {}
dict_from_pairs = dict([('a', 1), ('b', 2)])

Accessing & Modifying

value = dct['a']           # Access value
dct['d'] = 4              # Add new key-value pair
dct['a'] = 10             # Modify value
del dct['b']              # Delete key-value pair

Dictionary Methods

keys = dct.keys()          # Get all keys
values = dct.values()      # Get all values
items = dct.items()        # Get key-value pairs
default = dct.get('x', 0)  # Get with default

# loop through dict
for key, value in dct.items():
    print(key, value)

# check if key exists in dict
if 'x' in dct:
    print(dct['x'])
else:
    print('x not found')

DefaultDict

A defaultdict provides default values for missing keys.

from collections import defaultdict
dd = defaultdict(int)  # Default value is 0
dd['x'] += 1          # Works without key error

dl = defaultdict(list)
dl['x'].append(1) # Works without key error, creates empty list

OrderedDict

An OrderedDict remembers the order of items.

# A Python program to demonstrate working of OrderedDict
from collections import OrderedDict

od = OrderedDict()
od['a'] = 1
od['b'] = 2
od['c'] = 3
od['d'] = 4
od['a'] = 5

for key, value in od.items():
    print(key, value)

Equality in ordered dict

from collections import OrderedDict

# Create two ordered dictionaries with different orderings
od1 = OrderedDict([('a', 1), ('b', 2), ('c', 3)])
od2 = OrderedDict([('c', 3), ('b', 2), ('a', 1)])

# Compare the ordered dictionaries for equality
print(od1 == od2) # False

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Sets

Sets store unique elements and are unordered.

Creating Sets

s = {1, 2, 3, 4, 5}
empty_set = set()
set_from_list = set([1, 2, 3, 2, 1])  # Removes duplicates

Set Operations

s.add(6)            # Add an element
s.remove(3)         # Remove an element
union = s | {7, 8}  # Union
intersection = s & {2, 4, 6}  # Intersection
difference = s - {1, 2}  # Difference

Membership & Length

length = len(s)     # Get length
exists = 3 in s     # Check membership

# loop through set
for element in s:
    print(element)

int v/s tuple v/s set

v1 = (1) # int
v2 = (1,) # tuple
v3 = {1} # set

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Quick Functions Reference

Operation	Dictionary	Set
Length	len(dct)	len(s)
Access Value	dct[key]	❌
Add Item	dct[key] = value	s.add(x)
Remove Item	del dct[key]	s.remove(x)
Membership	key in dct	x in s
Union	❌	s | other_set
Intersection	❌	s & other_set
Difference	❌	s - other_set