Round

A number has a fractional part. For example 1.45 is between and 1 and 2 and we want to round it. With round() we can round it to 1 or 1.5.

To always round up, consider the math.ceil method. And to always round down, use math.floor. But to retain a fractional part, round() is helpful.

Round example

This receives one or two arguments. The second argument is optional. Round() returns a rounded number—we use it in an assignment.

Argument 1 This is the number we want to round. For this example, we are rounding the number 1.23456.

Argument 2 This tells how many numbers past the decimal point to keep. The final number is rounded up if the next digit is 5 or more.

number = 1.23456

# Use round built-in.
# ... This rounds up or down depending on the last digit.
print(round(number))
print(round(number, 0))
print(round(number, 1))
print(round(number, 2))
print(round(number, 3))1        0 digits
1.0      0 digits
1.2      1 digit
1.23     2 digits
1.235    3 digits, last one rounded up to 5

Round up, down

Let us consider this program. We use math.ceil to always round up to the nearest integer. Round() cannot do this—it will round up or down depending on the fractional value.

Info Ceil will always round up. So the ceil of 1.1 is 2. An integer is returned.

And Floor will round down to the nearest integer. The floor of 1.9 is 1. This is a mathematical function.

import math

number = 1.1

# Use math.ceil to round up.
result = math.ceil(number)
print(result)

# The round method will round down because 1.1 is less than 1.5.
print(round(number))2
1

Syntax notes

There is no math.round function in Python 3. Trying to invoke math.round will lead to an AttributeError. Instead, just use round().

Traceback (most recent call last):
  File "C:\programs\file.py", line 6, in <module>
    number = math.round(1.1)
AttributeError: 'module' object has no attribute 'round'

Math notes

Additional methods in Python can transform numbers as needed. The abs method provides absolute values. Built-in math methods make programs simpler.

A review

With round() in Python we gain a way to reduce the number of digits past the decimal place. Round increases or decreases a number based on the last digit.