Python Numbers and Operators: Arithmetic, Comparison and Logic

Arithmetic operators

print(7 + 3, 7 - 3, 7 * 3)
print(7 / 2)      # true division -> float
print(7 // 2)     # floor division -> int
print(7 % 2)      # remainder (modulo)
print(2 ** 10)    # power
print(-7 // 2)    # floors toward negative infinity
print(divmod(17, 5))

The two divisions confuse most beginners: / always produces a float even when the answer is whole (6 / 3 is 2.0), and // throws the fractional part away. The modulo % is how you test "is this number even?" — n % 2 == 0.

Assignment shortcuts

count = 0
count += 1      # count = count + 1
count *= 5
count -= 2
count //= 2
print(count)

text = "ab"
text += "c"     # works on strings and lists too
print(text)

Comparison operators

print(5 == 5, 5 != 3)
print(5 > 3, 5 < 3, 5 >= 5, 5 <= 4)
print(1 < 2 < 3)          # chained comparison
print(0.1 + 0.2 == 0.3)   # floating-point surprise
print(abs((0.1 + 0.2) - 0.3) < 1e-9)   # the safe way

Comparisons always produce True or False. Do not confuse == (equal?) with = (assign) — it is the most common syntax error in an if statement.

Logical operators: and, or, not

age, member = 20, True
print(age >= 18 and member)
print(age < 18 or member)
print(not member)

# short-circuit: the right side is skipped when not needed
x = 0
print(x != 0 and 10 / x > 1)    # no ZeroDivisionError

# and / or return the deciding value, not always a bool
print("" or "default")
print("value" and "second")

The last two lines show a useful trick: name = user_input or "Guest" gives a fallback when the input is empty.

Truthiness

Every value can be used where a boolean is expected. These are falsy: False, None, 0, 0.0, "", [], {}, set(). Everything else is truthy, which lets you write if items: instead of if len(items) > 0:.

Operator precedence

From highest to lowest: ** → unary - → * / // % → + - → comparisons → not → and → or. When in doubt, add parentheses; they cost nothing and make intent obvious.

print(2 + 3 * 4)       # 14, not 20
print((2 + 3) * 4)     # 20
print(-2 ** 2)         # -4  (power binds first)
print((-2) ** 2)       # 4
print(not True or True)  # True: not binds tighter than or

The math module and useful built-ins

import math
print(round(3.14159, 2), abs(-8), max(3, 9, 1), min([4, 2, 7]))
print(sum([1, 2, 3, 4]), pow(2, 8))
print(math.sqrt(16), math.floor(3.9), math.ceil(3.1))
print(math.pi, math.e)
print(math.gcd(12, 18), math.factorial(5))
print(int("ff", 16), bin(10), hex(255))

Experiment with operators in the app — the Coding Python app runs real Python 3 on your phone, with examples, quizzes and challenges built in.

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Now that you can compute and compare, the next lesson uses those results to make decisions with if, elif and else.

Frequently asked questions