How to Swap Two Numbers in Python (With and Without a Function)

To swap two numbers in Python, use tuple unpacking: a, b = b, a. It is one line, works for any type (integers, floats, strings, list items) and needs no temporary variable. This guide shows that and the classic alternatives (a temporary variable, arithmetic and XOR), explains how swapping works inside a function, and includes a complete program for swapping two numbers entered by the user.

All examples were run with Python 3.12.5 in the Windows Command Prompt. The Python docs explain the underlying feature under tuples and sequence unpacking.

Swap two numbers in one line

a = 5
b = 9
a, b = b, a          # swap in one line
print("a =", a, "b =", b)

Output:

a = 9 b = 5

The right side b, a is evaluated first and packed into a tuple (9, 5); then the tuple is unpacked into a and b. Because both values are read before anything is assigned, nothing is overwritten.

Four ways to swap two numbers

# 1. tuple unpacking (the Pythonic way)
a, b = 5, 9
a, b = b, a
print("tuple unpacking:", a, b)

# 2. a temporary variable
a, b = 5, 9
temp = a
a = b
b = temp
print("temp variable:  ", a, b)

# 3. arithmetic (+ and -)
a, b = 5, 9
a = a + b
b = a - b
a = a - b
print("arithmetic:     ", a, b)

# 4. bitwise XOR (integers only)
a, b = 5, 9
a ^= b
b ^= a
a ^= b
print("XOR:            ", a, b)

Output:

tuple unpacking: 9 5
temp variable:   9 5
arithmetic:      9 5
XOR:             9 5

Method	Code	Works for	Notes
Tuple unpacking	a, b = b, a	Any type	Recommended
Temporary variable	t = a; a = b; b = t	Any type	Works in every language
Arithmetic	a = a + b; b = a - b; a = a - b	Numbers	Floats can lose precision
XOR	a ^= b; b ^= a; a ^= b	Integers only	A classic interview trick

Why arithmetic swapping is risky with floats

a, b = 0.1, 0.7
a = a + b
b = a - b
a = a - b
print("arithmetic swap:", a, b)        # rounding errors creep in

a, b = 0.1, 0.7
a, b = b, a
print("tuple swap:     ", a, b)        # exact

x, y = "cat", "dog"
x, y = y, x                             # works for any type, not just numbers
print("strings:        ", x, y)

Output:

arithmetic swap: 0.7 0.09999999999999998
tuple swap:      0.7 0.1
strings:         dog cat

Swap two numbers using a function

A function cannot change the caller’s variables by rebinding its parameters, because Python passes references to objects, and assigning inside the function only changes the local names. Return the swapped values and assign them:

def swap(x, y):
    """Return the two values in reverse order."""
    return y, x


def swap_inside(x, y):
    x, y = y, x          # only swaps the LOCAL names x and y


a, b = 5, 9
swap_inside(a, b)
print("after swap_inside:", a, b)      # unchanged

a, b = swap(a, b)                       # assign the returned tuple
print("after swap:       ", a, b)

Output:

after swap_inside: 5 9
after swap:        9 5

Python program to swap two numbers entered by the user

a = input("Enter the first number: ")
b = input("Enter the second number: ")

a, b = float(a), float(b)
print(f"Before swapping: a = {a:g}, b = {b:g}")
a, b = b, a
print(f"After swapping:  a = {a:g}, b = {b:g}")

Output (the user typed 12 and 7.5):

Enter the first number: 12
Enter the second number: 7.5
Before swapping: a = 12, b = 7.5
After swapping:  a = 7.5, b = 12

Swap items in a list or dictionary

scores = [40, 75, 90, 60]
scores[0], scores[2] = scores[2], scores[0]      # swap two elements of a list
print(scores)

prices = {"apple": 1.2, "mango": 2.5}
prices["apple"], prices["mango"] = prices["mango"], prices["apple"]   # swap two dictionary values
print(prices)

Output:

[90, 75, 40, 60]
{'apple': 2.5, 'mango': 1.2}

How a, b = b, a works

The bytecode shows what really happens: in CPython 3.12 the compiler does not even build a tuple for a two-name swap. It loads b and a onto the stack and stores them back in the opposite order:

import dis

def swap(a, b):
    a, b = b, a
    return a, b

dis.dis(swap)          # show the bytecode behind a, b = b, a

Output:

  3           0 RESUME                   0

  4           2 LOAD_FAST                1 (b)
              4 LOAD_FAST                0 (a)
              6 STORE_FAST               1 (b)
              8 STORE_FAST               0 (a)

  5          10 LOAD_FAST                0 (a)
             12 LOAD_FAST                1 (b)
             14 BUILD_TUPLE              2
             16 RETURN_VALUE

More simple Python programs:

• Get the first and last digit of a number
• Python program to print prime numbers
• Convert a string to float in Python
• Reverse a number in Python

Frequently asked questions