Dictionaries

d1 = {"a": 1, "b": 2, "c": 3}​print(d1["a"])        # 1print(d1.get("b"))    # 2print(d1.get("z", 0)) # 0, default if key not found​# add new key-valued1["d"] = 4         ​# update existing keyd1["a"] = 10   ​# remove keydel d1["b"]

d = {"a": 1, "b": 0, "": 3}​# len(d) → number of key-value pairsprint("len(d):", len(d))   # 3​# in → check if key existsprint("'a' in d:", "a" in d)   # Trueprint("'z' in d:", "z" in d)   # False​# all(d) → True if all keys are truthy# Here we have an empty string "" as a key → falsyprint("all(d):", all(d))   # False​# any(d) → True if any key is truthyprint("any(d):", any(d))   # True, because "a" and "b" are truthy keys​# sorted(d) → returns sorted list of keysprint("sorted(d):", sorted(d))  # ['', 'a', 'b']

# Dictionary Methods Demod = {"a": 1, "b": 2, "c": 3}​# keys(), values(), items()print("Keys:", d.keys())        # dict_keys(['a', 'b', 'c'])print("Values:", d.values())    # dict_values([1, 2, 3])print("Items:", d.items())      # dict_items([('a', 1), ('b', 2), ('c', 3)])​# get(key, default)print("get('a'):", d.get("a"))         # 1print("get('z', 0):", d.get("z", 0))   # 0 (default)​# pop(key)val = d.pop("b")print("pop('b'):", val, "| dict now:", d)   # 2, dict without 'b'​# popitem() (removes last inserted item)key, val = d.popitem()print("popitem():", (key, val), "| dict now:", d)​# setdefault(key, default)print("setdefault('x', 42):", d.setdefault("x", 42))print("After setdefault:", d)​# update(other)d.update({"y": 100, "z": 200})print("After update:", d)​# copy()d_copy = d.copy()print("Copy of dict:", d_copy)​# clear()d.clear()print("After clear:", d)

# Existing dictionariesdict({"a": 1, "b": 2})   # {'a': 1, 'b': 2}​# Keyword argumentsdict(a=1, b=2)   # {'a': 1, 'b': 2}​# Sequence of pairs (iterable of 2-element iterables)dict([("a", 1), ("b", 2)])   # {'a': 1, 'b': 2}dict([["x", 10], ["y", 20]]) # {'x': 10, 'y': 20}​# Zipped iterableskeys = ["a", "b", "c"]values = [1, 2, 3]dict(zip(keys, values))   # {'a': 1, 'b': 2, 'c': 3}

d1 = {"a": 1, "b": 2}d2 = {"b": 2, "a": 1}d3 = {"a": 1, "b": 3}​print(d1 == d2)   # True (same key-value pairs, order irrelevant)print(d1 == d3)   # False (value for 'b' differs)print(d1 != d3)   # True

d1 = {"a": 1}d2 = {"a": 2}​print(d1 < d2)  # TypeError: '<' not supported between instances of 'dict' and 'dict'

print(sorted(d1.items()) < sorted(d2.items()))  # Works, compares lists of tuples

{key_expr: value_expr for item in iterable if condition}

nums = [1, 2, 3, 4, 5]​# ----------------------------------------# Basic dict comprehensionsquares = {x: x**2 for x in nums}print("Number -> Square:", squares)​# ----------------------------------------# Filtering itemsevens = {x: x**2 for x in nums if x % 2 == 0}print("Even -> Square:", evens)​# ----------------------------------------# Conditional expressionslabels = {x: ("even" if x % 2 == 0 else "odd") for x in nums}print("Number -> Even/Odd label:", labels)​# ----------------------------------------# Transforming stringsword = "comprehension"char_map = {ch: ord(ch) for ch in word}print("Char -> ASCII code:", char_map)​# Deduplicate automatically (dict keys must be unique)upper_map = {ch.lower(): ch.upper() for ch in word}print("Lower -> Upper (unique keys):", upper_map)​# ----------------------------------------# Nested loops in dict comprehensionpairs = {(x, y): x * y for x in range(1, 4) for y in range(1, 4)}print("Pairs -> Product:", pairs)​# ----------------------------------------# Swapping keys and valuesfruit_prices = {"apple": 1.2, "banana": 0.5, "pear": 0.8}swapped = {price: fruit for fruit, price in fruit_prices.items()}print("Price -> Fruit:", swapped)​# ----------------------------------------# Complex conditionstudent_scores = {"Alice": 85, "Bob": 60, "Charlie": 95}passed = {name: score for name, score in student_scores.items() if score >= 70}print("Passed students:", passed)​# ----------------------------------------# Using functionsdef cube(x): return x**3cubes = {x: cube(x) for x in nums}print("Number -> Cube:", cubes)​# ----------------------------------------# Real-world example: word counttext = "the quick brown fox jumps over the lazy dog the fox"word_count = {word: text.split().count(word) for word in set(text.split())}print("Word counts:", word_count)

# --- Creating dictionaries ---empty = {}d1 = {"a": 1, "b": 2, "c": 3}d2 = dict(x=10, y=20)nested = {"Alice": {"phone": "123", "email": "alice@example.com"}}print("Empty dict:", empty)print("d1:", d1)print("d2:", d2)print("Nested dict:", nested)​# --- Accessing values ---print("d1['a']:", d1["a"])print("d1.get('b'):", d1.get("b"))print("d1.get('z', 0):", d1.get("z", 0))  # default​# --- Modifying dictionaries ---d1["d"] = 4              # add new key-valued1["a"] = 10             # update existingprint("After add/update:", d1)​del d1["b"]              # delete keyprint("After del 'b':", d1)​val = d1.pop("c")        # remove and return valueprint("pop('c'):", val, "| dict now:", d1)​d1.update({"e": 50, "f": 60})  # add multipleprint("After update:", d1)​d1.setdefault("g", 70)   # insert only if key doesn't existprint("After setdefault('g', 70):", d1)​d1_copy = d1.copy()      # shallow copyprint("Copy:", d1_copy)​d1.clear()print("After clear:", d1)​# --- Dictionary Methods Demo on new dict ---d = {"a": 1, "b": 2, "c": 3}print("Keys:", d.keys())print("Values:", d.values())print("Items:", d.items())last_item = d.popitem()  # removes last inserted itemprint("popitem():", last_item, "| dict now:", d)​# --- Built-in functions ---print("Length:", len(d))                  # 2 after popitemprint("'a' in d:", 'a' in d)             # Trueprint("All keys truthy?", all(d))        # Trueprint("Any key truthy?", any(d))         # Trueprint("Sorted keys:", sorted(d))         # ['b', 'c']​# --- Iterating over dictionaries ---for k in d:    print("Key:", k)for k, v in d.items():    print("Key-Value:", k, v)for v in d.values():    print("Value:", v)​# --- Nested and complex dictionaries ---grades = {"Alice": [90, 85], "Bob": [75, 80]}print("Grades:", grades)contacts = {    "Alice": {"phone": "123", "email": "alice@example.com"},    "Bob": {"phone": "456", "email": "bob@example.com"}}print("Contacts:", contacts)​# --- Dictionary comprehension ---squares = {x: x**2 for x in range(5)}print("Squares dict:", squares)​# --- Using dictionaries in loops ---for student, scores in grades.items():    print(student, "average:", sum(scores)/len(scores))