unit-syntax adds support for physical units to the Python language:

>>> speed = 5 meters/second
>>> (2 seconds) * speed
10 meter

Why? I like to use Python as a calculator for physical problems and wished it had the type safety of explicit units along with the readability of normal notation.

unit-syntax works in Jupyter notebooks, standalone Python scripts, and Python packages.

How does it work?

Getting Started

Install the package:

$ pip install unit-syntax

... with Jupyter/IPython

To enable unit-syntax in a Jupyter/IPython session run:

%load_ext unit_syntax

Tip: In Jupyter this must be run in its own cell before any units expressions are evaluated.

... with standalone scripts

To run a standalone script with units:

$ python -m unit_syntax <path_to_script.py>

Note that this installs a custom import hook that affects all imports performed by the script.

... with Python packages

To use/distribute a package with unit-syntax, add this in your __init__.py:

from unit_syntax.import_hook import enable_units_for_package
enable_units_for_package(__name__)

This applies the transform only to sub-modules of your package.

Usage

An interactive notebook to play around with units

Units can be applied to any "simple" expression:

• number: 1 meter
• variables: x parsec, y.z watts, area[id] meters**2
• lists and tuples: [1., 37.] newton meters
• unary operators: -x dBm
• power: x**2 meters

In expressions mixing units and binary operators, parenthesize:

one_lux = (1 lumen)/(1 meter**2)

Units can be used in any place where Python allows expressions, e.g:

• function arguments: area_of_circle(radius=1 meter)
• list comprehensions: [x meters for x in range(10)]

Quantities can be converted to another measurement system:

>>> (88 miles / hour) furlongs / fortnight
236543.5269120001 furlong / fortnight
>>> (0 degC) degF
31.999999999999936 degree_Fahrenheit

Compound units (e.g. newtons/meter**2) are supported and follow the usual precedence rules.

Units may not begin with parentheses (consider the possible interpretations of x (meters)). Parentheses are allowed anywhere else:

# parsed as a function call, will result in a runtime error
x (newton meters)/(second*kg)
# a-ok
x newton meters/(second*kg)

Using unknown units produces a syntax error at import time:

>>> 1 smoot
...
SyntaxError: 'smoot' is not defined in the unit registry

How does it work?

unit-syntax transforms python-with-units into standard python that calls the excellent pint units handling library.

The parser is pegen, which is a standalone version of the same parser generator used by Python itself. The grammar is a lightly modified version the official Python grammar shipped with pegen.

Syntax transformation in IPython/Jupyter uses IPython custom input transformers.

Syntax transformation of arbitrary Python modules uses importlib's MetaPathFinder, see import-transforms and unit_syntax.import_hook for details.

Why only allow units on simple expressions?

Imagine units were instead parsed as operator with high precedence and you wrote this reasonable looking expression:

ppi = 300 pixels/inch
y = x inches * ppi

inches * ppi would be parsed as the unit, leading to (at best) a runtime error sometime later and at worst an incorrect calculation. This could be avoided by parenthesizing the expression (e.g. (x inches) * ppi, but if that's optional it's easy to forget. So the intent of this restriction is to make these risky forms uncommon and thus more obvious. This is not a hypothetical concern, I hit this within 10 minutes of first using the initial syntax.

Prior Art

The immediate inspriration of unit-syntax is a language called Fortress from Sun Microsystems. Fortress was intended as a modern Fortran, and had first-class support for units in both the syntax and type system.

F# (an OCaml derivative from Microsoft) also has first class support for units.

The Julia package Unitful.jl

A long discussion on the python-ideas mailing list about literal units in Python.

Development

To regenerate the parser:

python -m pegen python_units.gram -o unit_syntax/parser.py

Running tests:

$ poetry install --with dev
$ poetry run pytest

Future work and open questions

• Test against various ipython and python versions
• Ensure bytecode caching still works
• Test with wider range of source files with the wildcard loader
• Unit type hints, maybe checked with @runtime_checkable. More Pint typechecking discussion
• Typography of output
• pre-parse units
• talk to pint about interop between UnitRegistries
• Fix reported location of SyntaxError when paren missing