Dictionaries in Python

# Empty dictionary
empty1 = {}
empty2 = dict()

# With initial values
person = {
    "name": "John",
    "age": 30,
    "city": "NYC"
}

# Using dict() constructor
d1 = dict(name="Alice", age=25)
d2 = dict([("a", 1), ("b", 2)])  # From list of tuples
d3 = dict(zip(["a", "b"], [1, 2]))  # From two lists

# From keys with default value
keys = ["a", "b", "c"]
d = dict.fromkeys(keys, 0)  # {"a": 0, "b": 0, "c": 0}

# Valid keys
d = {
    "string": 1,      # String
    42: "number",     # Integer
    (1, 2): "tuple",  # Tuple
    True: "boolean",  # Boolean
}

# Invalid keys (will raise TypeError)
# d = {[1, 2]: "list"}     # Lists are mutable
# d = {{"a": 1}: "dict"}   # Dicts are mutable

person = {"name": "John", "age": 30}

# Direct access (raises KeyError if missing)
print(person["name"])  # "John"
# print(person["email"])  # KeyError!

# Using get() - safe access with default
print(person.get("name"))         # "John"
print(person.get("email"))        # None
print(person.get("email", "N/A")) # "N/A"

# Check if key exists
if "name" in person:
    print(person["name"])

# setdefault - get or set default
person.setdefault("country", "USA")  # Sets if missing, returns value

person = {"name": "John"}

# Add or update single key
person["age"] = 30
person["age"] = 31  # Update

# Update multiple keys
person.update({"city": "NYC", "job": "Dev"})
person.update(country="USA")  # Keyword args

# Python 3.9+ merge operators
d1 = {"a": 1}
d2 = {"b": 2}
merged = d1 | d2           # {"a": 1, "b": 2}
d1 |= d2                   # In-place merge

# Remove keys
del person["age"]                    # Raises KeyError if missing
age = person.pop("age", None)        # Safe remove with default
item = person.popitem()              # Remove and return last item
person.clear()                       # Remove all items

person = {"name": "John", "age": 30, "city": "NYC"}

# Views are dynamic
keys = person.keys()      # dict_keys(['name', 'age', 'city'])
values = person.values()  # dict_values(['John', 30, 'NYC'])
items = person.items()    # dict_items([...])

# Iterate
for key in person:
    print(key)

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

# Basic syntax: {key_expr: value_expr for item in iterable}
squares = {x: x**2 for x in range(5)}
# {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

# With condition
evens = {x: x**2 for x in range(10) if x % 2 == 0}
# {0: 0, 2: 4, 4: 16, 6: 36, 8: 64}

# Swap keys and values
original = {"a": 1, "b": 2}
swapped = {v: k for k, v in original.items()}
# {1: "a", 2: "b"}

# From two lists
keys = ["name", "age"]
values = ["John", 30]
person = {k: v for k, v in zip(keys, values)}

# Manual counting
text = "hello"
counts = {}
for char in text:
    counts[char] = counts.get(char, 0) + 1
# {'h': 1, 'e': 1, 'l': 2, 'o': 1}

# Using Counter (preferred)
from collections import Counter
counts = Counter(text)
counts.most_common(2)  # [('l', 2), ('h', 1)]

# Manual grouping
words = ["apple", "bat", "car", "ant", "bee"]
by_letter = {}
for word in words:
    first = word[0]
    by_letter.setdefault(first, []).append(word)
# {'a': ['apple', 'ant'], 'b': ['bat', 'bee'], 'c': ['car']}

# Using defaultdict (preferred)
from collections import defaultdict
by_letter = defaultdict(list)
for word in words:
    by_letter[word[0]].append(word)

users = {
    "user1": {"name": "Alice", "age": 25},
    "user2": {"name": "Bob", "age": 30}
}

# Access nested values
users["user1"]["name"]  # "Alice"

# Safe nested access
users.get("user3", {}).get("name", "Unknown")