Reverse a number in Python

Reversing 12345 gives 54321. The program takes the digits off the end of the number one by one and builds the new number from them.

The program

n = int(input("Enter a number: "))

number = abs(n)
reversed_number = 0
while number > 0:
    last_digit = number % 10
    reversed_number = reversed_number * 10 + last_digit
    number //= 10

if n < 0:
    reversed_number = -reversed_number

print(f"Reversed: {reversed_number}")

Typed in this example: 12345. Press Run to type your own.

Enter a number: 12345
Reversed: 54321

How it works

• number % 10 gives the last digit, and number //= 10 removes it.
• reversed_number * 10 + last_digit moves the digits already collected one place to the left and adds the new digit at the end. The values go 5, 54, 543, 5432, 54321.
• The program reverses abs(n) and adds the minus sign back at the end, so -123 becomes -321.
• Zeros at the end disappear: 1200 reversed is 21, not 0021, because a whole number cannot start with zeros.

Another way: string slicing

for n in [12345, -123, 1200]:
    text = str(abs(n))[::-1]
    reversed_number = int(text)
    if n < 0:
        reversed_number = -reversed_number
    print(n, "->", reversed_number)

12345 -> 54321
-123 -> -321
1200 -> 21

[::-1] is a slice with a step of -1. It reads the string from the end to the start. The reverse a string page explains it in more detail. Without abs(), str(-123)[::-1] would be "321-", and int() would raise a ValueError.