Python #2 — Variables, Data Types & Strings

# Type hints — optional but recommended
name: str = "Alice"
age: int = 25
price: float = 9.99
is_active: bool = True

def greet(name: str) -> str:
    return f"Hello, {name}!"

# Booleans are a subclass of int in Python
print(True + True)   # 2
print(True * 5)      # 5
print(False + 1)     # 1

# Truthy and falsy values
# Falsy: False, 0, 0.0, "", [], {}, set(), None
# Everything else is truthy

if []:            print("empty list is truthy")   # doesn't run
if [0]:           print("list with item is truthy") # runs
if "":            print("empty string is truthy")  # doesn't run
if "0":           print("non-empty string is truthy") # runs

# None is Python's null
result = None

if result is None:   # use 'is', not '=='
    print("No result yet")

# Common pattern — default argument
def find_user(id: int, default=None):
    # ... look up user ...
    return None  # not found

text = "Hello, Python!"

# text[start:stop:step]
print(text[0:5])    # Hello  (index 0 to 4)
print(text[7:])     # Python! (from index 7 to end)
print(text[:5])     # Hello  (start to index 4)
print(text[-7:])    # Python! (last 7 chars)
print(text[::2])    # Hlo yhn (every 2nd char)
print(text[::-1])   # !nohtyP ,olleH (reversed!)

name = "Rupa"
score = 98.5

# 1. f-strings (Python 3.6+) — use this
print(f"Name: {name}, Score: {score:.1f}")  # Name: Rupa, Score: 98.5

# 2. .format() — older, still common
print("Name: {}, Score: {:.1f}".format(name, score))

# 3. % operator — legacy, avoid
print("Name: %s, Score: %.1f" % (name, score))

pi = 3.14159265

print(f"{pi:.2f}")     # 3.14      — 2 decimal places
print(f"{pi:10.2f}")   #       3.14 — right-aligned, width 10
print(f"{1234567:,}")  # 1,234,567 — thousand separator
print(f"{0.75:.0%}")   # 75%       — percentage
print(f"{'hi':>10}")   #         hi — right-align string
print(f"{'hi':<10}")   # hi         — left-align string
print(f"{'hi':^10}")   #     hi     — center string

# Triple quotes — whitespace is preserved
sql = """
    SELECT *
    FROM products
    WHERE price < 100
    ORDER BY name
"""

# Dedented (remove leading whitespace)
from textwrap import dedent
query = dedent("""
    SELECT *
    FROM products
""").strip()

s = "  Hello, World!  "

# Cleaning
s.strip()           # "Hello, World!"    remove whitespace both ends
s.lstrip()          # "Hello, World!  "  left only
s.rstrip()          #  "  Hello, World!" right only

# Case
s.lower()           # "  hello, world!  "
s.upper()           # "  HELLO, WORLD!  "
s.title()           # "  Hello, World!  "
s.capitalize()      # "  hello, world!  " (only first char)

# Search
"World" in s                 # True
s.find("World")              # 9 (index) or -1 if not found
s.count("l")                 # 3
s.startswith("  Hello")      # True
s.endswith("!  ")            # True

# Modify (strings are immutable — these return new strings)
s.replace("World", "Python") # "  Hello, Python!  "
", ".join(["a", "b", "c"])  # "a, b, c"
"a,b,c".split(",")          # ["a", "b", "c"]

# Explicit conversion
int("42")         # 42
float("3.14")     # 3.14
str(100)          # "100"
bool(0)           # False
bool("hello")     # True
list("abc")       # ['a', 'b', 'c']

# Careful with invalid conversions
int("hello")      # ❌ ValueError
int("3.14")       # ❌ ValueError — use float() then int()
int(float("3.14")) # ✅ 3