Working with Dictionaries

Beginner Python
dictget()items()keys()values()defaultdictCounter

Objective

Create and manipulate Python dictionaries, use safe access patterns, and leverage specialized dict types.

Tools & Technologies

  • dict
  • get()
  • items()
  • keys()
  • values()
  • defaultdict
  • Counter

Key Commands

d.get(key, default)
d.items()
d.update(other)
collections.defaultdict(list)
Counter(lst).most_common(10)

Architecture Overview

graph TD DICT[dict = key:value pairs] DICT --> ACCESS[Access\nd[key]\nd.get(key, default)] DICT --> MUTATE[Mutate\nd[key] = val\ndel d[key]] DICT --> ITER[Iterate\nd.keys()\nd.values()\nd.items()] DICT --> MERGE[Merge\nd.update(other)\nd2 = d1 | d2 Python 3.9+] style DICT fill:#1a1a2e,stroke:#00d4ff,color:#e0e0e0

Step-by-Step Process

01
Create and Access

Build dicts and access values safely.

# Literal syntax
person = {'name': 'Alice', 'age': 25, 'city': 'Toronto'}

# Access
print(person['name'])           # KeyError if missing
print(person.get('age'))        # None if missing
print(person.get('job', 'N/A')) # default if missing

# Check existence
'name' in person   # True
'job' in person    # False
02
Iterate and Modify

Loop over dicts and update values.

# Iterate
for key, value in person.items():
    print(f'{key}: {value}')

# Modify
person['age'] = 26
person.update({'job': 'developer', 'age': 27})

# Dict comprehension
squares = {n: n**2 for n in range(1, 6)}

# Merge (Python 3.9+)
d3 = d1 | d2
03
Specialized Dict Types

Use defaultdict and Counter for common patterns.

from collections import defaultdict, Counter

# defaultdict: no KeyError for missing keys
wordcount = defaultdict(int)
for word in text.split():
    wordcount[word] += 1

# Counter: frequency counting
from collections import Counter
words = ['apple', 'banana', 'apple', 'cherry', 'apple']
c = Counter(words)
print(c.most_common(2))  # [('apple', 3), ('banana', 1)]

Challenges & Solutions

  • d['key'] raises KeyError; d.get('key') returns None — always use get() for user-supplied keys
  • Dict order is preserved (insertion order) since Python 3.7

Key Takeaways

  • Counter is the fastest way to count occurrences of items
  • defaultdict eliminates 'if key not in d' boilerplate