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Dictionaries

Dictionaries store key-value pairs — the most powerful and commonly used Python data structure.

Python Dictionaries

Dictionaries store data in key-value pairs. Keys must be unique and immutable. Dictionaries are ordered (Python 3.7+) and mutable.

Syntax

<pre><code># Creating dicts
person = {"name": "Alice", "age": 25, "city": "NYC"}
empty = {}
also_empty = dict()

# Accessing values
print(person["name"])          # Alice
print(person.get("age"))       # 25
print(person.get("salary", 0)) # 0 (default if not found)

# Modifying
person["age"] = 26              # Update
person["email"] = "a@mail.com"  # Add
del person["city"]              # Delete
popped = person.pop("email")    # Remove and return

# Iteration
for key in person:
    print(key)

for value in person.values():
    print(value)

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

# Dict methods
print(person.keys())   # dict_keys
print(person.values()) # dict_values
print(len(person))     # Number of keys

# Nested dict
student = {
    "name": "Bob",
    "grades": {"math": 95, "english": 87}
}
print(student["grades"]["math"])  # 95

# Dict comprehension (preview)
squares = {x: x**2 for x in range(5)}
</code></pre>

Revision Notes

• {key: value} syntax; keys must be unique+immutable
• Access: dict[key] or dict.get(key, default)
• .keys(), .values(), .items() for iteration
• del or .pop() to remove
• Ordered since Python 3.7
• Nested dicts for hierarchical data

Word Frequency Counter

Medium

Write a function word_frequency(text) that takes a string and returns a dictionary mapping each word (lowercase) to the number of times it appears.

Input:

"apple banana apple orange banana apple"

Output:

{'apple': 3, 'banana': 2, 'orange': 1}

Show Hint

Loop through the words, and for each word use dict.get(word, 0) + 1 to increment its count.

Solve this Challenge

Show Solution

def word_frequency(text):
    counts = {}
    for word in text.lower().split():
        counts[word] = counts.get(word, 0) + 1
    return counts

print(word_frequency("apple banana apple orange banana apple"))

Sets Back to Course

Dictionaries

Python Dictionaries

Syntax

Revision Notes

Dictionaries Quiz

Word Frequency Counter