`decimal`

The Python decimal module provides support for correctly rounded decimal floating-point arithmetic. It offers several advantages over the built-in floating-point type, including preserving significant digits.

This module allows you to make precise calculations in finance and other fields that require accurate decimal representation.

Here’s a quick example:

>>> from decimal import Decimal
>>> Decimal("0.1") + Decimal("0.2")
Decimal('0.3')

>>> # Using the built-in float
>>> 0.1 + 0.2
0.30000000000000004

Try it with your own numbers to see what each type actually stores:

Most decimal fractions have no exact binary form, so float rounds to the nearest value it can hold, while Decimal keeps the digits you wrote.

Key Features

Provides a Decimal data type for decimal floating-point arithmetic
Supports arbitrary precision arithmetic
Offers configurable rounding modes
Supports exact decimal representation of numbers
Provides context objects for controlling precision and rounding

Frequently Used Classes and Functions

Object	Type	Description
`decimal.Decimal`	Class	Creates decimal floating-point numbers
`decimal.getcontext()`	Function	Gets the current default context
`decimal.setcontext()`	Function	Sets the current default context
`decimal.Context`	Class	Encapsulates precision and rounding settings
`decimal.localcontext()`	Function	Returns a context manager that will set the current context

Rounding modes:

Mode	Rounds
`decimal.ROUND_CEILING`	Towards positive infinity
`decimal.ROUND_DOWN`	Towards zero
`decimal.ROUND_FLOOR`	Towards negative infinity
`decimal.ROUND_HALF_DOWN`	To nearest, with ties rounding toward zero
`decimal.ROUND_HALF_EVEN`	To nearest, with ties going to nearest even integer
`decimal.ROUND_HALF_UP`	To nearest, with ties rounding away from zero
`decimal.ROUND_UP`	Away from zero
`decimal.ROUND_05UP`	Away from zero if the last digit after rounding toward zero would have been 0 or 5. Otherwise, rounds toward zero

Examples

Creating a Decimal object for precise arithmetic:

>>> from decimal import Decimal
>>> Decimal("1.1") + Decimal("2.2")
Decimal('3.3')

Setting precision for calculations:

>>> from decimal import Decimal, localcontext

>>> with localcontext(prec=42):
...     Decimal("1") / Decimal("42")
...
Decimal('0.0238095238095238095238095238095238095238095')

>>> Decimal("1") / Decimal("42")
Decimal('0.02380952380952380952380952381')

Common Use Cases

Performing precise financial calculations
Handling user input that requires exact decimal representation
Avoiding rounding errors in scientific computations
Implementing configurable rounding strategies

Real-World Example

Suppose you need to calculate the total cost of items in a shopping cart with precise financial calculations:

>>> from decimal import Decimal

>>> prices = [Decimal("19.99"), Decimal("9.99"), Decimal("4.99")]
>>> total = sum(prices)
>>> total
Decimal('34.97')

This example demonstrates how the decimal module can be used to sum up monetary values accurately, avoiding the pitfalls of binary floating-point representation.

Tutorial

Numbers in Python

In this tutorial, you'll learn about numbers and basic math in Python. You'll explore integer, floating-point numbers, and complex numbers and see how perform calculations using Python's arithmetic operators, math functions, and number methods.

basics python

For additional information on related topics, take a look at the following resources: