Python — Dictionaries

Creating dictionaries

Dictionaries store data as key-value pairs:

person = {
    "name": "Ada",
    "age": 36,
    "job": "Engineer"
}

# Empty dictionary
empty = {}

# From a list of tuples
pairs = [("name", "Ada"), ("age", 36)]
person = dict(pairs)

# Using dict() constructor
person = dict(name="Ada", age=36)

Keys must be immutable — strings, numbers, tuples. Values can be anything.

Accessing values

person = {"name": "Ada", "age": 36, "job": "Engineer"}

# Bracket notation
print(person["name"])  # Ada

# .get() — safe access with default
print(person.get("name"))       # Ada
print(person.get("email"))      # None
print(person.get("email", "N/A"))  # N/A

.get() is safer than brackets — it returns None (or a default) instead of raising KeyError.

Modifying dictionaries

person = {"name": "Ada", "age": 36}

# Add or update a key
person["email"] = "ada@example.com"
person["age"] = 37

print(person)  # {'name': 'Ada', 'age': 37, 'email': 'ada@example.com'}

# Update multiple keys at once
person.update({"job": "Engineer", "city": "London"})

Removing elements

person = {"name": "Ada", "age": 36, "job": "Engineer"}

# pop — remove and return
age = person.pop("age")
print(age)     # 36
print(person)  # {'name': 'Ada', 'job': 'Engineer'}

# pop with default
email = person.pop("email", "not found")

# del — remove by key
del person["job"]

# popitem — remove last inserted item
last = person.popitem()

# clear — empty the dictionary
person.clear()

Dictionary methods

person = {"name": "Ada", "age": 36, "job": "Engineer"}

# keys, values, items
print(person.keys())    # dict_keys(['name', 'age', 'job'])
print(person.values())  # dict_values(['Ada', 36, 'Engineer'])
print(person.items())   # dict_items([('name', 'Ada'), ('age', 36), ('job', 'Engineer')])

# Convert to lists
names = list(person.keys())
values = list(person.values())
pairs = list(person.items())

# Check if key exists
print("name" in person)      # True
print("email" in person)     # False

# Length
print(len(person))           # 3

# Copy
copy = person.copy()

Iterating over dictionaries

person = {"name": "Ada", "age": 36, "job": "Engineer"}

# Iterate over keys (default)
for key in person:
    print(key, person[key])

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

# Iterate over values
for value in person.values():
    print(value)

Dictionary comprehensions

# Create a dictionary from a list
names = ["Ada", "Grace", "Linus"]
name_lengths = {name: len(name) for name in names}
print(name_lengths)  # {'Ada': 3, 'Grace': 5, 'Linus': 5}

# Filter
scores = {"Alice": 90, "Bob": 75, "Charlie": 85, "Diana": 95}
high_scores = {name: score for name, score in scores.items() if score >= 85}
print(high_scores)  # {'Alice': 90, 'Charlie': 85, 'Diana': 95}

# Transform values
doubled = {k: v * 2 for k, v in scores.items()}

Nested dictionaries

Dictionaries can contain other dictionaries:

students = {
    "ada": {
        "name": "Ada Lovelace",
        "age": 36,
        "grades": {"math": 95, "science": 88}
    },
    "grace": {
        "name": "Grace Hopper",
        "age": 85,
        "grades": {"math": 92, "science": 90}
    }
}

# Access nested values
print(students["ada"]["grades"]["math"])  # 95

Common patterns

Counting with defaultdict

from collections import defaultdict

words = "the cat sat on the mat the cat".split()
counts = defaultdict(int)

for word in words:
    counts[word] += 1

print(dict(counts))  # {'the': 3, 'cat': 2, 'sat': 1, 'on': 1, 'mat': 1}

Grouping with defaultdict

from collections import defaultdict

students = [
    ("Alice", "A"), ("Bob", "B"), ("Charlie", "A"),
    ("Diana", "B"), ("Eve", "A")
]

groups = defaultdict(list)
for name, grade in students:
    groups[grade].append(name)

print(dict(groups))  # {'A': ['Alice', 'Charlie', 'Eve'], 'B': ['Bob', 'Diana']}

Merging dictionaries

# Python 3.9+
dict1 = {"a": 1, "b": 2}
dict2 = {"b": 3, "c": 4}
merged = dict1 | dict2  # {'a': 1, 'b': 3, 'c': 4}

# Python 3.5+
merged = {**dict1, **dict2}

Dictionary as a switch/case

def get_day_type(day):
    types = {
        "Monday": "Weekday",
        "Tuesday": "Weekday",
        "Wednesday": "Weekday",
        "Thursday": "Weekday",
        "Friday": "Weekday",
        "Saturday": "Weekend",
        "Sunday": "Weekend"
    }
    return types.get(day, "Invalid day")

print(get_day_type("Monday"))  # Weekday

The setdefault method

# setdefault — get value or set default if key missing
config = {}
config.setdefault("host", "localhost")
config.setdefault("port", 8080)
print(config)  # {'host': 'localhost', 'port': 8080}

# Useful for building lists per key
word_index = {}
for i, word in enumerate(["a", "b", "a", "c", "b", "a"]):
    word_index.setdefault(word, []).append(i)

print(word_index)  # {'a': [0, 2, 5], 'b': [1, 4], 'c': [3]}

Mini Practice

1. Create a dictionary of five countries and their capitals
2. Use .get() to safely access a key that might not exist
3. Write a dictionary comprehension that maps numbers 1-10 to their squares
4. Count the frequency of each letter in a sentence using a dictionary
5. Merge two dictionaries and handle duplicate keys by summing values

Next: conditional statements →