unitx

Physical units, scalar quantities, and dimensional analysis for Python, with exact rational unit scales and exponents.

unitx is in early development. Public APIs may change during the 0.x series.

Installation

Requires Python 3.12 or later, with no mandatory runtime dependencies.

Install a published release:

python -m pip install unitx

To use the current development version, run this from the repository root:

python -m pip install -e .

This README describes the current source tree, including unreleased changes. For a published release, consult the README at its matching Git tag.

Upgrading from 0.3.0

Version 0.4.0 replaces the implementation previously published on PyPI as 0.3.0. It includes incompatible changes:

• Python 3.12 or later is required; Python 3.11 is no longer supported.

• Import classes from their modules. The former top-level exports and unitx.utils namespace are no longer provided:

  from unitx.dimension import DIMENSIONLESS, Dimension
  from unitx.quantity import Quantity
  from unitx.unit import Unit
  

• Unit no longer defines len() or special truth-value behavior. Use unit == Unit('') to check the formal identity, or unit.is_dimensionless() to check physical dimensions; these are different questions.

• Conversion and arithmetic can produce different magnitude types, especially for Decimal inputs. Review the numerical behavior before relying on a result's concrete type or approximate equality.

Quick start

Import the classes from their modules:

>>> from unitx.quantity import Quantity
>>> from unitx.unit import Unit
>>> distance = 3 * Unit('m')
>>> elapsed = Quantity(2, 's')
>>> speed = distance / elapsed
>>> print(speed)
1.5 m/s
>>> print(speed.to('cm/s'))
150.0 cm/s

Quantity(value, unit) accepts a unit expression or an existing Unit object. Multiplying a scalar by a unit constructs a quantity; dividing a scalar by a unit constructs a quantity with the reciprocal unit. Construction preserves the original magnitude and its type.

Conversion and arithmetic

Use .to() to convert between units with the same physical dimensions. Addition and subtraction convert the right operand to the left operand's unit:

>>> length = Quantity(1, 'm')
>>> print(length + Quantity(50, 'cm'))
3/2 m
>>> length.to('cm').value
100
>>> Quantity(1, 'cm').to('m').value
Fraction(1, 100)

Multiplication and division combine units. Powers apply to both the magnitude and the unit. Multiplication or division by a Unit alone preserves the magnitude unchanged.

Conversion, addition, subtraction, and ordering across incompatible dimensions raise unitx.exceptions.DimensionError. Equality across incompatible dimensions returns False.

For repeated conversions, reuse a target Unit object to avoid parsing the expression on every call.

Unit expressions

Expressions support registered symbols, singular English names, SI prefixes, multiplication, division, parentheses, and rational powers:

>>> Unit('kg m/s2') == Unit('kg*m/s**2')
True
>>> Unit('m^(1/2)') ** 2 == Unit('m')
True
>>> Unit('micrometre') == Unit('um')
True
>>> Unit('ms') == Unit('m s')
False

• Multiplication can use * or whitespace. Adjacent letters form one unit token: ms is a millisecond, while m s is metre times second.
• Powers can use adjacent numbers, ^, or **. Exponents written as ratios require parentheses, as in m^(1/2); decimal exponents such as m^0.5 are exact.
• Multiplication and division associate left to right. Use m/(s A) to put both s and A in the denominator; m/s A means (m/s)*A.
• Unit symbols and names are case-sensitive. u, µ, and μ are accepted micro prefixes. Use lowercase k for kilo; K denotes kelvin.

The current unit table contains s, m, g, A, K, mol, and cd, plus the dimensionless identity. Mass scales are referenced to kilograms: g has factor 1/1000, and kg has factor 1. Named derived units such as N are not registered yet; use expressions such as kg*m/s2.

Units, dimensions, and equality

A Unit retains its formal factors. A Quantity compares physical values across compatible units:

>>> Unit('m') == Unit('cm')
False
>>> Quantity(1, 'm') == Quantity(100, 'cm')
True
>>> len({Quantity(1, 'm'), Quantity(100, 'cm')})
1
>>> from unitx.dimension import Dimension
>>> speed.dimension == Dimension(L=1, T=-1)
True

Equal quantities have equal hashes when their magnitudes are hashable. Different formal unit expressions can have the same physical dimension and scale without being equal as Unit objects.

An expression can be dimensionless and still carry a scale. Plain numbers added to or subtracted from dimensionless quantities represent unscaled values:

>>> scaled = Quantity(1, 'km/m')
>>> scaled.is_dimensionless()
True
>>> scaled == 1000
True
>>> (scaled + 1).to('').value
Fraction(1001, 1)

Numerical behavior

Exact unit algebra and numerical magnitude evaluation have different rules:

• Rational unit scales and exponents remain exact. Prefer Fraction or Decimal for exact exponents; float exponents may be approximated with an InexactFloatWarning.
• Conversion and arithmetic can change the magnitude's type. Finite Decimal operands are converted exactly to Fraction, without Decimal context rounding.
• Other magnitudes use native numerical arithmetic. In particular, integer division produces a float, and nonintegral magnitude powers can round or produce complex values.
• Equality is exact, without a tolerance. Finite floats are compared as their stored binary rational values. Floating-point conversions and irrational powers may round; compare their values approximately in a common unit when appropriate.

>>> from decimal import Decimal
>>> from fractions import Fraction
>>> Quantity(Decimal('0.1'), 'm').to('cm').value
10
>>> (Quantity(Fraction(1), 'm') / Quantity(3, 's')).value
Fraction(1, 3)
>>> Quantity(0.1, '') == Quantity(Fraction(1, 10), '')
False

The built-in integer factorization backend is bounded. Unresolved factors emit IncompleteFactorWarning and can limit simplification and comparisons. SymPy is an optional, more capable factorization backend:

python -m pip install sympy

Current scope

• NumPy scalars, arrays, and NumPy operations are not supported yet. NumPy integration is planned. Structural compatibility with a magnitude protocol does not imply NumPy support.
• Custom magnitudes must support scalar arithmetic, return a Python bool from equality, and defer raw operations with Unit operands to its reflected operators.
• Scalar multiplication and division of quantities currently accept int, float, Fraction, and Decimal. Complex magnitudes are supported, including results of fractional powers; ordering follows the magnitude's native rules.
• Unit conversion is multiplicative. Offset temperature scales such as Celsius and Fahrenheit are not supported.

Building blocks

The package includes inline type annotations and a py.typed marker. Construction and operations with Unit preserve the magnitude type in type hints; conversion and numerical arithmetic use result unions where the concrete type depends on runtime values or units. Modules whose names start with _ are internal.

Development

Install the development version and test dependencies:

python -m pip install -e ".[test]"
python -m pytest -q
python -m ruff check .
python -m ruff format --check .

The pytest suite includes doctests and Pyright checks for the typed API. CI runs the suite with both the built-in and SymPy factorization backends.

Releases

The release workflow validates the source, builds a wheel and source distribution, checks their metadata, and tests the installed wheel. Manual workflow runs store the distributions without uploading them to PyPI.

Publishing a GitHub release runs the same checks and uploads the distributions to PyPI. The release tag must match the project version, for example v0.4.0 for version 0.4.0. Configure a PyPI Trusted Publisher for the repository, workflow release.yml, and GitHub environment pypi before publishing.

See CHANGELOG.md for release history. Licensed under the MIT License.