Dictionaries & Sets

# Creating dictionaries
user = {
    "name":   "Nelson Njihia",
    "age":    26,
    "role":   "engineer",
    "skills": ["Python", "JavaScript", "SQL"],
}

# Accessing values
user["name"]           # "Nelson Njihia"
user.get("email")      # None (safe — no KeyError)
user.get("email", "")  # "" (default value)

# Modifying
user["email"]  = "nelson@nelsonlabs.dev"   # add or update
del user["age"]                            # delete key

# Iterating
for key in user:                           # keys
    print(key)
for value in user.values():               # values
    print(value)
for key, value in user.items():           # key-value pairs
    print(f"{key}: {value}")

# Dictionary comprehension
squares = {n: n ** 2 for n in range(1, 6)}
# {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

# Merging dictionaries (Python 3.9+)
defaults = {"theme": "dark", "lang": "en"}
settings = {**defaults, "lang": "sw", "font_size": 16}
# or: settings = defaults | {"lang": "sw"}

# Creating sets
tags    = {"python", "django", "web"}
numbers = {1, 2, 3, 4, 5}

# Sets ignore duplicates
unique = {1, 2, 2, 3, 3, 3}  # {1, 2, 3}

# From a list — removes duplicates
courses = ["Python", "Python", "Django", "Flask"]
unique_courses = set(courses)  # {"Python", "Django", "Flask"}

# Membership — O(1) time (instant regardless of size)
"python" in tags   # True
"rust" in tags     # False

# Set operations
a = {1, 2, 3, 4}
b = {3, 4, 5, 6}

a | b   # {1, 2, 3, 4, 5, 6}  — union (all elements)
a & b   # {3, 4}              — intersection (shared)
a - b   # {1, 2}              — difference (in a, not in b)
a ^ b   # {1, 2, 5, 6}        — symmetric difference

# Modifying
tags.add("flask")
tags.discard("web")  # remove if present (no error if missing)