Operators, Expressions & Strings

n % 2 == 0            # is n even?
n % 5 == 0            # is n a multiple of 5?
total % 60            # leftover seconds after taking out whole minutes
1234 % 10             # 4  - the last digit
1234 // 10            # 123 - everything except the last digit
(i + 1) % 7           # cycle 0,1,2,3,4,5,6,0,1,2... forever

total_seconds = 9045

hours = total_seconds // 3600            # 2
remaining = total_seconds % 3600         # 1845
minutes = remaining // 60                # 30
seconds = remaining % 60                 # 45

print(f"{hours:02d}:{minutes:02d}:{seconds:02d}")

7 == 7      # True   - equal to
7 != 3      # True   - not equal to
7 > 3       # True
7 <= 7      # True

# Chained comparisons read exactly like maths
score = 85
print(0 <= score <= 100)      # True

count = 0
total = 10

# Without short-circuiting this would crash with ZeroDivisionError.
# Because count != 0 is False, Python never evaluates the division.
if count != 0 and total / count > 5:
    print("high average")
else:
    print("no data or low average")

bool(0)        # False
bool(0.0)      # False
bool("")       # False   - empty string
bool([])       # False   - empty list
bool({})       # False   - empty dict
bool(None)     # False

bool(-1)       # True    - any non-zero number
bool("0")      # True    - a non-empty string, even if it says "0"
bool("False")  # True    - still just text
bool(" ")      # True    - a space is a character

total = 10
total += 5     # same as total = total + 5   ->  15
total -= 3     # 12
total *= 2     # 24
total //= 5    # 4
total **= 2    # 16
print(total)

word = "PYTHON"

print(word[0])     # 'P'   first character
print(word[5])     # 'N'   sixth character
print(word[-1])    # 'N'   last character
print(word[-2])    # 'O'   second from the end
print(len(word))   # 6
print(word[6])     # IndexError - there is no position 6

s = "PROGRAMMING"

s[0:4]      # 'PROG'   positions 0,1,2,3
s[:4]       # 'PROG'   start defaults to 0
s[4:]       # 'RAMMING'  stop defaults to the end
s[:]        # whole string
s[-4:]      # 'MING'   last four
s[:-4]      # 'PROGRAM'  everything except the last four
s[::2]      # 'PORMIG'  every second character
s[::-1]     # 'GNIMMARGORP'  reversed
s[20:30]    # ''  out of range slicing gives an empty string, no error

name = "amin"
name[0] = "A"       # TypeError: 'str' object does not support item assignment

name = "amin"

name.upper()          # this computes "AMIN" and throws it away
print(name)           # 'amin'  - nothing changed

name = name.upper()   # THIS is how you keep the result
print(name)           # 'AMIN' 

raw = "  jOHN   smith  "

cleaned = raw.strip()          # 'jOHN   smith'
parts = cleaned.split()        # ['jOHN', 'smith']   <- split() with no argument
                               #    splits on ANY run of whitespace
first = parts[0].title()       # 'John'
last = parts[1].title()        # 'Smith'

print(f"{last}, {first}")      # 'Smith, John'
print("-".join(parts))         # 'jOHN-smith' 

# password_strength.py
password = "Sunshine2026"

long_enough = len(password) >= 8
has_digit = any(ch.isdigit() for ch in password)
has_upper = password != password.lower()
has_lower = password != password.upper()

score = long_enough + has_digit + has_upper + has_lower   # booleans add as 1 and 0

print(f"Password: {password}")
print(f"  8+ characters : {long_enough}")
print(f"  has a digit   : {has_digit}")
print(f"  has upper     : {has_upper}")
print(f"  has lower     : {has_lower}")
print(f"  score         : {score}/4")