Operators & Expressions

Basic Arithmetic

# Addition
5 + 3      # 8
5.5 + 2.5  # 8.0

# Subtraction
10 - 4     # 6

# Multiplication
4 * 3      # 12
'ha' * 3   # 'hahaha' (string repetition)

# Division (always returns float)
10 / 3     # 3.3333333333333335
10 / 5     # 2.0 (still float!)

# Floor Division (integer division)
10 // 3    # 3
-10 // 3   # -4 (rounds toward negative infinity)

# Modulo (remainder)
10 % 3     # 1
-10 % 3    # 2 (sign follows divisor)

# Exponentiation
2 ** 10    # 1024
9 ** 0.5   # 3.0 (square root)

Division Gotchas

# ⚠️ Floor division vs truncation
import math

-7 // 2    # -4 (floor division)
math.trunc(-7 / 2)  # -3 (truncation)

# ⚠️ Modulo with negative numbers
-7 % 2     # 1 (not -1!)
# Because: -7 = (-4) * 2 + 1

Operator Precedence (High to Low)

Equality and Identity

# Equality (==) compares values
5 == 5        # True
'5' == 5      # False (different types)
[1, 2] == [1, 2]  # True

# Identity (is) compares memory addresses
a = [1, 2, 3]
b = [1, 2, 3]
a == b   # True (same value)
a is b   # False (different objects)

a = b    # Now a references same object as b
a is b   # True

Chained Comparisons

# Python allows chaining
x = 5

# Instead of:
if x > 0 and x < 10:
    pass

# You can write:
if 0 < x < 10:
    pass

# Works with any comparison
if 0 <= x < y <= 100:
    pass

Comparison Operators Table

# a = 5, b = 3

a == b  # False (equal)
a != b  # True (not equal)
a > b   # True (greater than)
a < b   # False (less than)
a >= b  # True (greater or equal)
a <= b  # False (less or equal)

Comparing Different Types

# ⚠️ Comparing different types
'hello' > 5   # TypeError in Python 3
              # (Python 2 would return True)

# None comparisons
None == None  # True
None is None  # True (preferred)
None < 5      # TypeError

Comparing Strings

# Lexicographic comparison
'apple' < 'banana'  # True
'abc' < 'abd'      # True
'abc' < 'ab'       # False (longer string is 'greater')

# Case matters
'Apple' < 'apple'  # True (uppercase < lowercase)

Logical Operators

# and - returns first falsy or last value
True and True    # True
True and False   # False
False and True   # False
False and False  # False

# or - returns first truthy or last value
True or True     # True
True or False    # True
False or True    # True
False or False   # False

# not - returns opposite boolean
not True    # False
not False   # True

Short-Circuit Evaluation

# 'and' short-circuits on first falsy
def expensive_func():
    print('Called!')
    return True

False and expensive_func()  # 'Called!' is NOT printed

# 'or' short-circuits on first truthy
True or expensive_func()    # 'Called!' is NOT printed

# Common pattern: default values
name = user_input or 'Anonymous'

# Another pattern: guard clauses
if items and items[0] == 'special':
    process(items[0])

Walrus Operator (:=)

# Assignment expression (Python 3.8+)
import random

# Instead of:
num = random.randint(1, 10)
if num > 5:
    print(f'Big number: {num}')

# You can write:
if (num := random.randint(1, 10)) > 5:
    print(f'Big number: {num}')

# Useful in while loops
while (line := input('Enter: ')) != 'quit':
    print(f'You entered: {line}')

Assignment Operators

x = 10

x += 5    # x = x + 5  → 15
x -= 3    # x = x - 3  → 12
x *= 2    # x = x * 2  → 24
x /= 4    # x = x / 4  → 6.0
x //= 2   # x = x // 2 → 3.0
x %= 2    # x = x % 2  → 1.0
x **= 3   # x = x ** 3 → 1.0

# Bitwise assignment
x = 0b1010
x &= 0b1100  # x = x & 0b1100
x |= 0b0001  # x = x | 0b0001
x ^= 0b1111  # x = x ^ 0b1111
x >>= 1      # x = x >> 1
x <<= 2      # x = x << 2

Identity Operators

# is / is not - compare memory addresses
a = [1, 2, 3]
b = [1, 2, 3]
c = a

a is b      # False (different objects)
a is c      # True (same object)
a is not b  # True

# Use 'is' for singletons
if x is None:
    pass
if x is not None:
    pass