How to reverse a number in Python?

Reversing an integer number is a common programming task. You may need to reverse a number for palindrome checking, mathematical operations, or data processing scenarios.

Input: 12345
Output: 54321

There are two main approaches to reverse a number in Python:

• Convert the number into a string, reverse the string and reconvert it into an integer

• Reverse mathematically without converting into a string

Using String Slicing

This method converts the number to a string, reverses it using Python's slice notation [::-1], and converts back to integer ?

def reverse_string_method(num):
    st = str(num)
    revst = st[::-1]
    ans = int(revst)
    return ans

num = 12345
print(reverse_string_method(num))

54321

One-liner Version

num = 12345
reversed_num = int(str(num)[::-1])
print(reversed_num)

54321

Mathematical Approach

This method uses mathematical operations to extract digits from right to left and build the reversed number ?

def reverse_mathematical(num):
    rev = 0
    while num > 0:
        digit = num % 10  # Extract last digit
        rev = (rev * 10) + digit  # Build reversed number
        num = num // 10  # Remove last digit
    return rev

num = 12345
print(reverse_mathematical(num))

54321

How It Works

The mathematical method works by:

1. Extracting the last digit using modulo operator (%)
2. Adding it to the reversed number after shifting existing digits left
3. Removing the last digit using integer division (//)
4. Repeating until all digits are processed

Handling Negative Numbers

Both methods need modification to handle negative numbers ?

def reverse_with_negative(num):
    is_negative = num < 0
    num = abs(num)  # Work with positive number
    
    # Reverse using string method
    reversed_num = int(str(num)[::-1])
    
    return -reversed_num if is_negative else reversed_num

# Test with positive and negative numbers
print(reverse_with_negative(12345))   # 54321
print(reverse_with_negative(-12345))  # -54321

54321
-54321

Comparison

Conclusion

Use string slicing for simplicity and readability. Choose the mathematical approach when you need to avoid string conversion or want a pure arithmetic solution.