Lesson 8.2: Dictionary Methods

1. Viewing Keys, Values, and Items

Python provides three methods to view the contents of a dictionary:

fruit_prices = {
    "apple": 1.20,
    "banana": 0.50,
    "cherry": 2.00
}

# Get all keys
print(fruit_prices.keys())

# Get all values
print(fruit_prices.values())

# Get all key-value pairs as tuples
print(fruit_prices.items())

# Convert to a regular list if needed
key_list = list(fruit_prices.keys())
print(key_list)

2. Merging with update()

The update() method adds key-value pairs from one dictionary to another. If a key already exists, its value is updated:

settings = {
    "theme": "light",
    "font_size": 14,
    "language": "English"
}

new_settings = {
    "theme": "dark",
    "notifications": True
}

settings.update(new_settings)
print(settings)

Notice that "theme" was updated from "light" to "dark", and "notifications" was added as a new key.

3. Removing with pop() and clear()

scores = {"Alice": 95, "Bob": 87, "Carol": 92}

# pop() removes a key and returns its value
bob_score = scores.pop("Bob")
print(f"Bob's score was: {bob_score}")
print(scores)

# pop() with a default value (no error if key missing)
dave_score = scores.pop("Dave", "Not found")
print(f"Dave's score: {dave_score}")

# clear() removes ALL entries
scores.clear()
print(scores)

4. Iterating Over Dictionaries

You can loop through dictionaries in several ways:

Loop Over Keys (Default)

ages = {"Alice": 25, "Bob": 30, "Carol": 28}

# Looping over a dict gives you the keys
for name in ages:
    print(name)

Loop Over Key-Value Pairs with items()

# Most common pattern: loop over items()
for name, age in ages.items():
    print(f"{name} is {age} years old")

Loop Over Values Only

# Sum all the ages
total = 0
for age in ages.values():
    total += age
print(f"Total of all ages: {total}")

sentence = "the cat sat on the mat the cat"
word_count = {}

for word in sentence.split():
    if word in word_count:
        word_count[word] += 1
    else:
        word_count[word] = 1

for word, count in word_count.items():
    print(f"'{word}' appears {count} time(s)")

5. Nested Dictionaries

Dictionary values can themselves be dictionaries, creating nested structures perfect for complex data:

students = {
    "alice": {
        "age": 20,
        "major": "Math",
        "gpa": 3.8
    },
    "bob": {
        "age": 22,
        "major": "CS",
        "gpa": 3.5
    }
}

# Access nested values with chained brackets
print(students["alice"]["major"])
print(students["bob"]["gpa"])

# Loop through nested data
for name, info in students.items():
    print(f"{name}: {info['major']} major, GPA: {info['gpa']}")

6. Other Useful Methods

data = {"a": 1, "b": 2, "c": 3}

# len() returns the number of key-value pairs
print(len(data))

# setdefault() returns value if key exists,
# otherwise inserts key with default and returns it
val = data.setdefault("d", 4)
print(val)
print(data)

# copy() creates a shallow copy
data_copy = data.copy()
print(data_copy)