dictionaries.md

Python by Example: Dictionaries

Dictionaries store key-value pairs—like a real dictionary maps words to definitions. Keys must be hashable (strings, numbers, tuples); values can be anything. Use them when you need fast lookup by name or ID. Lookup is typically O(1). Curly braces {} create dicts; colons separate keys from values.

What you'll learn:

• Creating and accessing dicts
• .get() for safe access with a default
• Adding and updating keys
• Iterating with .items()

# Create a dict
person = {"name": "Alice", "age": 30, "city": "Paris"}
print(person)

# Access by key
print(person["name"])

# Safe access with .get()
print(person.get("country", "unknown"))

# Add or update
person["email"] = "alice@example.com"
person["age"] = 31
print(person)

# Keys, values, items
print(person.keys())
print(person.values())
for k, v in person.items():
    print(k, v)

person["name"] raises KeyError if the key is missing. person.get("country", "unknown") returns "unknown" instead. Use .get() when the key might not exist.

To run this program:

$ python source/dictionaries.py
{'name': 'Alice', 'age': 30, 'city': 'Paris'}
Alice
unknown
{'name': 'Alice', 'age': 31, 'city': 'Paris', 'email': 'alice@example.com'}
dict_keys(['name', 'age', 'city', 'email'])
dict_values(['Alice', 31, 'Paris', 'alice@example.com'])
name Alice
age 31
city Paris
email alice@example.com

Tip: Prefer .get(key, default) over dict[key] when the key might be missing—it avoids crashes.

Try it: Create a dict of your favorite things (e.g., color, food) and print one value.

Source: dictionaries.py

Next: Sets