Add Two Numbers

Adding two numbers is one of the first arithmetic operations you need to understand in Python. Python supports several approaches: the plain + operator for the simplest cases, the built-in sum() function when working with a collection, and functools.reduce() for functional-style code. This page covers all three, plus the important step of converting user input to a numeric type before adding.

Method 1: Using the + Operator

The addition operator + is the most direct way to add two numbers. It works with integers, floats, and complex numbers.

The same operator works with floats without any changes:

x = 3.5
y = 1.2
print(x + y)   # Output: 4.7

Variable naming tip: avoid naming a variable sum — it shadows Python's built-in sum() function and can cause confusing bugs later in your code.

Method 2: Adding Numbers from User Input

When a user types a number, Python receives it as a string. You must convert it to an integer with int() or to a floating-point number with float() before you can do arithmetic. See Python Casting for a full explanation of type conversion.

a = int(input("Enter first number: "))
b = int(input("Enter second number: "))
result = a + b
print("The sum is:", result)

If the input might contain decimals, use float() instead of int():

a = float(input("Enter first number: "))
b = float(input("Enter second number: "))
print("The sum is:", a + b)

Method 3: Using the Built-in sum() Function

Python's built-in sum() function adds every item in an iterable (such as a list or tuple). It is the right tool when you have more than two numbers to add.

sum() also accepts an optional start argument, which is added to the result:

total = sum([1, 2, 3], 10)   # 10 + 1 + 2 + 3
print(total)                  # Output: 16

This is useful when you want to accumulate into a running total.

Method 4: Using functools.reduce()

reduce() from the functools module applies a two-argument function repeatedly across a sequence, reducing it to a single value. For simple addition sum() is clearer, but reduce() is valuable when the combining function is more complex than +.

You can also pass a lambda instead of a named function:

from functools import reduce

total = reduce(lambda x, y: x + y, [10, 20, 30, 40, 50])
print(total)   # Output: 150

Choosing the Right Approach

Related Topics

• Python Variables — how to store and name values
• Python Operators — full reference for arithmetic, comparison, and logical operators
• Python Numbers — integers, floats, and complex numbers explained
• Python Casting — converting between types with int(), float(), and str()
• Python User Input — reading values from the keyboard with input()