Python Dictionaries – The Complete Guide

# Literal syntax (most common)
person = {
    "name": "Alice",
    "age": 30,
    "city": "New York",
    "is_student": False
}

# Empty dictionary
empty_dict = {}

# Using dict() constructor
car = dict(make="Toyota", model="Camry", year=2022)

# From a list of pairs
pairs = [("a", 1), ("b", 2), ("c", 3)]
from_pairs = dict(pairs)

print(person)
print(car)
print(from_pairs)
print(f"Number of keys: {len(person)}")

product = {
    "name": "Laptop",
    "brand": "TechBrand",
    "price": 999.99,
    "in_stock": True
}

# Direct access (raises KeyError if missing)
print(product["name"])      # Laptop
print(product["price"])     # 999.99

# Safe access with .get() - returns None if missing
print(product.get("brand"))          # TechBrand
print(product.get("color"))          # None
print(product.get("color", "N/A"))   # N/A  (default value)

# Check if a key exists
if "in_stock" in product:
    print("Stock info available")

# Check if key NOT in dict
if "discount" not in product:
    print("No discount key")

student = {"name": "Bob", "age": 20}

# Add a new key
student["grade"] = "A"
student["major"] = "Computer Science"
print(student)

# Update an existing key (overwrites the value)
student["age"] = 21
print("After update:", student)

# Update multiple keys at once with .update()
student.update({
    "year": 2,
    "gpa": 3.8,
    "age": 22   # also updates existing key
})
print("After .update():", student)

config = {
    "host": "localhost",
    "port": 8080,
    "debug": True,
    "timeout": 30,
    "retries": 3
}

# del - raises KeyError if key not found
del config["debug"]
print(config)

# .pop() - removes and returns the value; optional default
timeout = config.pop("timeout")
print(f"Removed timeout: {timeout}")

missing = config.pop("nonexistent", "default_val")
print(f"Pop missing key: {missing}")

# .popitem() - removes and returns the last inserted pair (Python 3.7+)
last_key, last_val = config.popitem()
print(f"Popped last item: {last_key} = {last_val}")

# .clear() - empties the dictionary
temp = {"a": 1, "b": 2}
temp.clear()
print("After clear:", temp)

scores = {
    "Alice": 95,
    "Bob": 82,
    "Carol": 91,
    "David": 78
}

# Iterate keys (default)
print("Students:")
for name in scores:
    print(f"  {name}")

# Iterate values
print("\nScores:")
for score in scores.values():
    print(f"  {score}")

# Iterate key-value pairs (most common)
print("\nFull report:")
for name, score in scores.items():
    grade = "Pass" if score >= 80 else "Fail"
    print(f"  {name:<8}: {score}  ({grade})")

# Find the top scorer
top = max(scores, key=lambda name: scores[name])
print(f"\nTop scorer: {top} with {scores[top]}")

# Group words by their first letter
words = ["apple", "avocado", "banana", "blueberry", "cherry", "apricot", "coconut"]

grouped = {}
for word in words:
    letter = word[0]
    grouped.setdefault(letter, []).append(word)

for letter, group in sorted(grouped.items()):
    print(f"  {letter}: {group}")

company = {
    "name": "TechCorp",
    "employees": {
        "E001": {
            "name": "Alice",
            "role": "Engineer",
            "salary": 95000
        },
        "E002": {
            "name": "Bob",
            "role": "Manager",
            "salary": 110000
        },
        "E003": {
            "name": "Carol",
            "role": "Designer",
            "salary": 85000
        }
    }
}

# Access nested values
print(company["name"])
print(company["employees"]["E001"]["name"])   # Alice
print(company["employees"]["E002"]["salary"])  # 110000

# Update a nested value
company["employees"]["E003"]["salary"] = 90000

# Add a new nested record
company["employees"]["E004"] = {
    "name": "David",
    "role": "DevOps",
    "salary": 100000
}

# Iterate nested dict
print("\nAll employees:")
for emp_id, info in company["employees"].items():
    print(f"  {emp_id}: {info['name']} ({info['role']}) — ${info['salary']:,}")

# Square each number as a key-value pair
squares = {n: n**2 for n in range(1, 6)}
print(squares)   # {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

# Filter: only include even squares
even_squares = {n: n**2 for n in range(1, 11) if n % 2 == 0}
print(even_squares)   # {2: 4, 4: 16, 6: 36, 8: 64, 10: 100}

# Invert a dictionary (swap keys and values)
original = {"a": 1, "b": 2, "c": 3}
inverted = {v: k for k, v in original.items()}
print(inverted)   # {1: 'a', 2: 'b', 3: 'c'}

# Build a word-length mapping
words = ["python", "is", "awesome", "programming"]
word_lengths = {word: len(word) for word in words}
print(word_lengths)

def word_frequency(text):
    """Count how many times each word appears in a text."""
    words = text.lower().split()
    freq = {}
    for word in words:
        # Remove basic punctuation
        word = word.strip(".,!?;:")
        freq[word] = freq.get(word, 0) + 1
    return freq

text = """Python is great. Python is easy to learn.
Python is used for data science and web development.
Learning Python is fun!"""

freq = word_frequency(text)

# Sort by frequency (most common first)
sorted_freq = dict(sorted(freq.items(), key=lambda item: item[1], reverse=True))

print("Word frequencies:")
for word, count in list(sorted_freq.items())[:8]:
    bar = "█" * count
    print(f"  {word:<12}: {count}  {bar}")