x = 42            # int
x = 3.14           # float
x = "hello"        # str
x = True           # bool
x = [1, 2, 3]       # list
x = (1, 2, 3)       # tuple
x = {"a": 1}        # dict
x = {1, 2, 3}       # set
x = None            # NoneType

type(42)            # <class 'int'>
type(3.14)          # <class 'float'>
type("hi")          # <class 'str'>
type([1, 2])        # <class 'list'>
type((1, 2))        # <class 'tuple'>
type({"a": 1})      # <class 'dict'>
type({1, 2})        # <class 'set'>
type(None)          # <class 'NoneType'>

s = "  Hello, World  "

s.upper()                # "  HELLO, WORLD  "
s.lower()                 # "  hello, world  "
s.strip()                 # "Hello, World"
s.split(",")               # ['  Hello', ' World  ']
"-".join(["a", "b", "c"])  # "a-b-c"
s.replace("Hello", "Hi")   # "  Hi, World  "
s.strip().startswith("Hello")  # True
s.strip().endswith("World")    # True

name = "Ada"
age = 36
f"{name} is {age} years old"   # "Ada is 36 years old"
f"{age * 2}"                    # "72"

text = "Python"
text[0]        # 'P'
text[-1]       # 'n'
text[0:3]      # 'Pyt'
text[::-1]     # 'nohtyP'  (reversed)

nums = [3, 1, 2]

nums.append(4)          # [3, 1, 2, 4]
nums.extend([5, 6])      # [3, 1, 2, 4, 5, 6]
nums.insert(0, 99)       # [99, 3, 1, 2, 4, 5, 6]
nums.remove(99)          # removes first matching value -> [3, 1, 2, 4, 5, 6]
nums.pop()                # removes & returns last item -> 6
nums.pop(0)                # removes & returns item at index -> 3
nums.sort()                 # sorts in place
sorted(nums)                 # returns a new sorted list, original untouched
nums.reverse()                # reverses in place

nums = [10, 20, 30, 40, 50]
nums[1:3]      # [20, 30]
nums[:2]       # [10, 20]
nums[2:]       # [30, 40, 50]
nums[::2]      # [10, 30, 50]
len(nums)      # 5

d = {"a": 1, "b": 2}

d.get("a")             # 1
d.get("z")              # None
d.get("z", 0)            # 0 (default if key missing)
list(d.keys())            # ['a', 'b']
list(d.values())           # [1, 2]
list(d.items())             # [('a', 1), ('b', 2)]
d.update({"c": 3})            # {'a': 1, 'b': 2, 'c': 3}
d.pop("a")                     # removes & returns 1 -> {'b': 2, 'c': 3}
d.setdefault("z", 100)           # sets 'z' to 100 if not present, returns value
"b" in d                          # True — membership checks keys

# list comprehension
squares = [n ** 2 for n in range(10)]

# with a filter
evens = [n for n in range(20) if n % 2 == 0]

# dict comprehension
lengths = {word: len(word) for word in ["a", "bb", "ccc"]}

# set comprehension
unique_lengths = {len(word) for word in ["a", "bb", "ccc", "dd"]}

# generator comprehension (lazy, memory-efficient)
gen = (n ** 2 for n in range(1000000))
next(gen)  # 0

# nested comprehension — flatten a matrix
matrix = [[1, 2], [3, 4], [5, 6]]
flat = [n for row in matrix for n in row]  # [1, 2, 3, 4, 5, 6]

# unpacking / swap
a, b = 1, 2
a, b = b, a

# enumerate — index + value
for i, value in enumerate(["x", "y", "z"]):
    print(i, value)

# zip — pair up iterables
names = ["Ada", "Grace"]
ages = [36, 85]
for name, age in zip(names, ages):
    print(name, age)

# *args and **kwargs
def f(*args, **kwargs):
    print(args)    # tuple of positional args
    print(kwargs)  # dict of keyword args

f(1, 2, x=3, y=4)

# ternary expression
status = "adult" if age >= 18 else "minor"

# reading a file safely
with open("data.txt") as f:
    contents = f.read()
# file is automatically closed, even if an exception occurs

# try / except / finally
try:
    value = int("not a number")
except ValueError:
    value = 0
finally:
    print("done")

# walrus operator — assign inside an expression
data = [1, 2, 3, 4, 5]
if (n := len(data)) > 3:
    print(f"list has {n} items")