Dictionaries

# Empty dict
empty = {}
empty = dict()

# With initial values
user = {
    "name": "Alice",
    "age": 25,
    "email": "alice@example.com"
}

# Using dict() constructor
dict(name="Bob", age=30)  # Keys become strings: {'name': 'Bob', 'age': 30}

# From pairs
dict([("a", 1), ("b", 2)])  # {'a': 1, 'b': 2}

# Dict comprehension
squares = {x: x ** 2 for x in range(5)}
# {0: 0, 1: 1, 2: 4, 3: 9, 4: 9}

user = {"name": "Alice", "age": 25}

# Access
user["name"]          # 'Alice'
user.get("name")      # 'Alice'
user.get("phone")     # None — no error
user.get("phone", "N/A")  # 'N/A' — default value

# Check key existence
"name" in user        # True
"phone" not in user   # True

# Modify
user["age"] = 26      # Update existing key
user["phone"] = "555-0123"  # Add new key

# Safe access without KeyError
# user["phone"]       # KeyError if key missing
user.get("phone")     # None

user = {"name": "Alice", "age": 25, "role": "admin"}

# Keys, values, items
user.keys()      # dict_keys(['name', 'age', 'role'])
user.values()    # dict_values(['Alice', 25, 'admin'])
user.items()     # dict_items([('name', 'Alice'), ('age', 25), ('role', 'admin')])

# These are views — they reflect changes to the dict
keys = user.keys()
user["email"] = "alice@example.com"
print(keys)      # dict_keys(['name', 'age', 'role', 'email'])

# Removing
removed = user.pop("role")     # Removes and returns value — 'admin'
last = user.popitem()          # Removes and returns last inserted item (Python 3.7+)
del user["age"]                # Removes key — KeyError if missing
user.clear()                   # Empty the dict

# Merging (Python 3.9+)
a = {"x": 1, "y": 2}
b = {"y": 3, "z": 4}
merged = a | b       # {'x': 1, 'y': 3, 'z': 4}
a |= b               # Updates a in place

user = {"name": "Alice", "age": 25, "role": "admin"}

# Keys (default)
for key in user:
    print(key)

# Values
for value in user.values():
    print(value)

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

# Sorted iteration
for key in sorted(user):
    print(f"{key}: {user[key]}")

# Count occurrences
text = "mississippi"
counts = {}
for char in text:
    counts[char] = counts.get(char, 0) + 1
# {'m': 1, 'i': 4, 's': 4, 'p': 2}

# Using collections.Counter
from collections import Counter
counts = Counter(text)
# Counter({'i': 4, 's': 4, 'p': 2, 'm': 1})

from collections import defaultdict

# Group words by first letter
words = ["apple", "banana", "avocado", "cherry", "blueberry"]
groups = defaultdict(list)
for word in words:
    groups[word[0]].append(word)

print(dict(groups))
# {'a': ['apple', 'avocado'], 'b': ['banana', 'blueberry'], 'c': ['cherry']}

from collections import defaultdict

# Automatically creates default for missing keys
dd = defaultdict(int)
dd["a"] += 1      # No KeyError — int() defaults to 0
print(dd["a"])    # 1
print(dd["b"])    # 0 — auto-created

# Other defaults
defaultdict(list)  # [] for missing keys
defaultdict(set)   # set() for missing keys
defaultdict(lambda: "N/A")  # Custom default

# Since Python 3.7, regular dicts preserve insertion order
# OrderedDict is still useful for equality comparisons

from collections import OrderedDict
od = OrderedDict()
od["a"] = 1
od["b"] = 2
od.move_to_end("a")  # Move 'a' to the end

# Basic
squares = {x: x ** 2 for x in range(5)}

# With condition
even_squares = {x: x ** 2 for x in range(10) if x % 2 == 0}

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

# Valid keys: int, float, str, tuple (with hashable elements), frozenset
# Invalid keys: list, set, dict (mutable — not hashable)

valid = {
    42: "integer",
    3.14: "float",
    "hello": "string",
    (1, 2): "tuple",
    frozenset({1, 2}): "frozenset",
}

# invalid = {[1, 2]: "list"}  # TypeError: unhashable type: 'list'