Operators, Expressions & Precedence

Operators and operands

An operator is a symbol that performs an operation; the values it works on are operands. In marks + 5, + is the operator and marks and 5 are operands. An operator with one operand (like -x or not x) is unary; with two it is binary.

Arithmetic operators

a, b = 17, 5
print(a + b, a - b, a * b)
print(a / b, a // b, a % b, a ** 2)
print(-17 // 5, -17 % 5)        # floor division rounds DOWN

Relational (comparison) operators

They compare two values and always give True or False.

a, b = 17, 5
print(a > b, a < b, a == 17, a != b, a >= 17, b <= 4)
print('apple' < 'banana', 'Zoo' < 'apple')   # compared by Unicode codes

Strings are compared letter by letter using their Unicode codes; capital letters (65–90) come before small letters (97–122), so 'Zoo' < 'apple' is True.

Assignment operators

A compound assignment like x += 5 is a short form of x = x + 5.

x = 10
x += 5;  print(x)
x -= 3;  print(x)
x *= 2;  print(x)
x //= 5; print(x)
x **= 3; print(x)
x %= 10; print(x)

Logical operators

age, has_id = 20, False
print(age >= 18 and has_id)    # both must be True
print(age >= 18 or has_id)     # any one True is enough
print(not has_id)
print(0 and 5, 3 and 5, 0 or 'default', '' or 'guest')

Short-circuit: and stops at the first false value; or stops at the first true value, and they return that value itself. That is why '' or 'guest' gives 'guest' — handy for default values.

Identity and membership operators

L = [1, 2, 3]
M = L
N = [1, 2, 3]
print(M is L, N is L)        # identity: same object?
print(N == L, N is not L)    # equality: same value?
print('a' in 'class', 5 in [1, 2, 3], 'z' not in 'python')

Bitwise operators

They work on the binary digits of integers: & AND, | OR, ^ XOR, ~ NOT, << left shift, >> right shift.

print(12 & 10, 12 | 10, 12 ^ 10)   # 1100 and 1010

12 = 1100 and 10 = 1010 → AND 1000 = 8, OR 1110 = 14, XOR 0110 = 6.

Expressions and precedence

An expression is a combination of values, variables and operators that Python evaluates to a single value, e.g. (a + b) / 2. When an expression has many operators, precedence decides which is done first, and associativity decides the order among operators of equal precedence (mostly left to right; ** is right to left).

print(2 + 3 * 4)        # * before +
print((2 + 3) * 4)      # brackets first
print(2 ** 3 ** 2)      # ** is right to left: 2 ** 9
print(10 - 4 - 3)       # left to right
print(100 / 10 * 2)     # left to right
print(5 + 4 > 8 and not 3 > 4)

Summary & practice

• / gives float, // floor quotient, % remainder, ** power.
• Comparisons give True/False; logical operators short-circuit.
• is checks identity, == checks value, in checks membership.
• Precedence: () → ** → unary → * / // % → + - → comparisons → not → and → or.