Note

Python - round() - Round numbers to specified decimal places

Overview of the Python `round()` function: How to round numbers in Python

round(number, ndigits) is a built-in function used for rounding numbers in Python.

number: The number to round (can be a float or int).

ndigits (optional): Specifies the number of decimal places to round to. It can be:

positive integer: Round to the specified number of decimal places.
negative integer: Round to the left of the decimal point.
None (default): Round to the nearest integer.

Take a look at the examples below:

Basic Usage of `round()`

a = 2.586
rounded_a = round(a, 2)
print(rounded_a)  # 2.59

Output:

2.59

REALLY Important Note: round() function rounds ties (.5 values) to the nearest even number. For example:

print(round(2.5))  # 2
print(round(3.5))  # 4
print(round(127.5))  # 128

Output:

2
4
128

Returned Type Matches the Input Type

Returned type corresponds with the provided argument, as shown below:

b = 3.0
rounded_b = round(b, 2)
print(rounded_b)  # 3.0
print(type(rounded_b))  # <class 'float'>

c = 3
rounded_c = round(c, 2)
print(rounded_c)  # 3
print(type(rounded_c))  # <class 'int'>

Output:

3.0
<class 'float'>
3
<class 'int'>

Using Negative `ndigits`

What is interesting in round() function, is that you can provide negative integers in place of the ndigits argument. Take a look at that:

d = 329.262
rounded_d = round(d, 1)
print(rounded_d)  # 329.3

rounded_d = round(d, 0)
print(rounded_d)  # 329.0

rounded_d = round(d, -1)
print(rounded_d)  # 330.0

rounded_d = round(d, -2)
print(rounded_d)  # 300.0

rounded_d = round(d, -3)
print(rounded_d)  # 0.0

Output:

As you can see providing negative integer as an ndigits argument results in rounding number "from the left side".

Rounding Without `ndigits`

Finally, if we don't provide an ndigits argument at all, we will always round the number to an integer type. And naturally, Python won't have any problem with rounding negative numbers as well:

e = -3.14159
rounded_e = round(e)
print(rounded_e)  # -3
print(type(rounded_e))  # <class 'int'>

Output:

-3
<class 'int'>

Key Points to Remember about the `round()` Function

Flexible Rounding: Works with both positive and negative values for number and ndigits.
Type Preservation: Returns a float or int, depending on the input type.
Negative ndigits: Rounds to positions to the left of the decimal point.
Default Behavior: Without ndigits, rounds to the nearest int.