How to Round a Number in Python (round() Function & Decimal)

Learn how to round floating-point numbers in Python. Understand Banker's rounding, custom decimal precision, and formatting floats for display.

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Explanation

Floating-point arithmetic in computers can sometimes lead to unexpected precision issues due to how binary systems store decimals (e.g., `0.1 + 0.2` evaluating to `0.30000000000000004`). When rendering numbers to users or preparing metrics, rounding numbers is a common requirement. Python provides several ways to achieve this.

The primary tool is the built-in `round(number, ndigits)` function. It takes a number and an optional precision count. If `ndigits` is omitted, it rounds to the nearest integer. If `ndigits` is provided, it rounds to that number of decimal places.

A key property of Python's `round()` function is that it uses "Banker's rounding" (round half to even). When a number is exactly halfway between two integers (e.g., `2.5` or `3.5`), Python rounds to the nearest *even* number. This means `round(2.5)` is `2`, and `round(3.5)` is `4`. While mathematically fair over large datasets to prevent upward bias, for precise financial calculations you should use the `decimal` module's quantization tools.

Step-by-Step Implementation

1
Call the built-in round(number) function to round a float to the nearest whole integer.
2
Pass a second argument (e.g., round(num, 2)) to specify the number of decimal places to keep.
3
Use the decimal module with ROUND_HALF_UP for strict financial rounding calculations.

Code Example

This script demonstrates rounding floats using the built-in round function, Banker's rounding examples, and the decimal module.

round_numbers.py

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from decimal import Decimal, ROUND_HALF_UP

val = 3.14159

# 1. Round to nearest integer
print("Nearest Integer:", round(val))

# 2. Round to 2 decimal places
print("Two decimals:", round(val, 2))

# 3. Banker's Rounding demonstration (rounds to nearest even)
print("round(2.5):", round(2.5)) # Nearest even is 2
print("round(3.5):", round(3.5)) # Nearest even is 4

# 4. Math rounding up (Traditional rounding) using decimal module
amount = Decimal("2.5")
rounded_up = amount.quantize(Decimal("1"), rounding=ROUND_HALF_UP)
print("Decimal ROUND_HALF_UP (2.5):", rounded_up)

Terminal Output

Nearest Integer: 3
Two decimals: 3.14
round(2.5): 2
round(3.5): 4
Decimal ROUND_HALF_UP (2.5): 3

Frequently Asked Questions

Why does round(2.5) return 2 instead of 3 in Python?

Python uses Banker's rounding, which rounds half numbers to the nearest even integer. This reduces statistical bias (rounding up half numbers always causes a slight upward drift over large datasets).

How do I round a number always up or always down?

Import the math module and use math.ceil(x) to round up to the next integer, or math.floor(x) to round down to the previous integer.

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