Creating and Accessing Dictionaries
A dictionary is an unordered collection of key-value pairs. Each key maps to a value, like a real-world dictionary maps words to definitions. Dictionaries are defined using curly braces {}.
Keys must be unique and immutable (strings, numbers, or tuples). Values can be anything, including other dictionaries or lists.
Example
# Creating a dictionary
person = {
"name": "Alice",
"age": 30,
"city": "New York",
"hobbies": ["reading", "coding"]
}
# Accessing values
print(person["name"]) # Alice
print(person.get("age")) # 30
print(person.get("email", "N/A")) # N/A (default if key missing)
# Adding and updating
person["email"] = "alice@example.com" # Add new key
person["age"] = 31 # Update existing key
# Removing entries
del person["city"] # Delete a key
removed = person.pop("email") # Remove and return value
print(removed) # alice@example.com
print(person)
# {'name': 'Alice', 'age': 31, 'hobbies': ['reading', 'coding']}- dict[key] — Access value (raises KeyError if key missing)
- dict.get(key, default) — Access value (returns default if key missing)
- dict[key] = value — Add or update a key-value pair
- del dict[key] — Delete a key-value pair
- dict.pop(key) — Remove a key and return its value
- key in dict — Check if a key exists
Try Dictionaries
JavaScript
person = {
"name": "Alice",
"age": 30,
"city": "New York"
}
print("Name:", person["name"])
print("Age:", person.get("age"))
person["email"] = "alice@example.com"
print("Added email:", person["email"])
print("Has city?", "city" in person)
print("Keys:", list(person.keys())) Notes
- Use .get() instead of bracket notation when you're not sure if a key exists. It avoids KeyError exceptions and lets you provide a default value.
Dictionary Methods and Iterating
Dictionaries provide several useful methods for working with keys, values, and items. You can iterate over a dictionary's keys, values, or both simultaneously.
Dictionary comprehensions let you create dictionaries concisely, similar to list comprehensions.
Example
student = {"name": "Bob", "math": 92, "science": 88, "english": 95}
# Useful methods
print(student.keys()) # dict_keys(['name', 'math', 'science', 'english'])
print(student.values()) # dict_values(['Bob', 92, 88, 95])
print(student.items()) # dict_items([('name', 'Bob'), ('math', 92), ...])
# Iterating over keys
for key in student:
print(f"{key}: {student[key]}")
# Iterating over key-value pairs
for key, value in student.items():
print(f"{key} = {value}")
# Dictionary comprehension
numbers = [1, 2, 3, 4, 5]
square_dict = {n: n**2 for n in numbers}
print(square_dict) # {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
# Merging dictionaries
defaults = {"color": "blue", "size": "medium"}
custom = {"color": "red", "weight": 10}
merged = {**defaults, **custom}
print(merged) # {'color': 'red', 'size': 'medium', 'weight': 10} Try Dictionary Iteration
JavaScript
scores = {"math": 92, "science": 88, "english": 95}
print("Subject scores:")
for subject, score in scores.items():
print(f" {subject}: {score}")
avg = sum(scores.values()) / len(scores)
print(f"Average: {avg:.1f}")
# Dictionary comprehension
squares = {n: n**2 for n in range(1, 6)}
print("Squares:", squares) Notes
- Since Python 3.7, dictionaries maintain insertion order. In earlier versions, order was not guaranteed.