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A package for making Python unit-aware.

Project description


Use Units in your Python Code

WARNING: This code is still under development. Unless you are invigorated by the thought of an extended correspondence with me, I would recommend against its use for any serious (read: paid) project.

Here's something we've all done before:

length1 = 42 # Meters
# ... 500 lines later

length2 = 12 # Inches
result = length1 + length2 # Oops


pip install python-pyunits

Units are a pain.

And any time we're computing with some sort of physical quantities, we almost certainly have to take them into account. Whenever I read (or write) this kind of code, I am amazed at how many subtle bugs crop up due to missing or incorrect unit conversions.

WolframAlpha can do it, so why can't Python?

Why indeed? We're used to scientific software and even search engines auto-magically handling unit conversions and dimensional analysis for us. Python is used for a lot of scientific applications, but sadly, this feature isn't built-in to the language.

Ladies and gentlemen, I give you PyUnits:

from examples import example_units as eu

length1 = eu.Meters(42)
length2 = eu.Inches(12)

print(length1 + length2) # Prints "42.3048 m"

Okay. So maybe you're not turned on by unit conversions. I can understand that. But guess what? PyUnits also understands dimensional analysis. In fact, it understands it at least as well as Randall Munroe does:

Dimensional Analysis

from examples import example_units as eu
from pyunits.compound_units import Mul, Div

CubicMeters = Mul(eu.Meters, Mul(eu.Meters, eu.Meters))
CubicIn = Mul(eu.Inches, Mul(eu.Inches, eu.Inches))
Pascals = Div(eu.Joules, CubicMeters)
MilesPerIn3 = Div(eu.Miles, CubicIn)

plank_energy = eu.Joules(1.956E9)
core_pressure = Pascals(3.6E11)
# Technically, this is the Prius C gas mileage. It seems that Priuses
# (Priui?) have improved somewhat since the comic was published...
# Note also that we don't really have the ability to do mpg directly yet,
# so we do mi / in^3 and divide by 231. (1 gal = 231 in^3)
prius_mileage = MilesPerIn3(46 / 231)
channel_width = eu.Kilometers(33.8)

pi = plank_energy / core_pressure * prius_mileage / channel_width

print(pi) # Prints 3.14

"But wait", I hear you cry. "That's a totally contrived example!" And it may well be. So let's try something a little more... practical:


import math

from examples import example_units as eu
from pyunits.compound_units import Mul

SquareMiles = Mul(eu.Miles, eu.Miles)
SquareMeters = Mul(eu.Meters, eu.Meters)

population_density = 18600 / SquareMiles(1.0)
sex_frequency = 80 / eu.Years(1.0)
sex_duration = eu.Minutes(30)

denom_value = math.pi * population_density * sex_frequency * sex_duration
fraction = 2.0 / denom_value

# Ensure result is in square meters.
fraction = SquareMeters(fraction)

# Square root has to be done outside of PyUnits, since we don't support that
# (yet).
sex_radius = math.sqrt(fraction.raw)
print(sex_radius) # Prints "139.33", the correct result for these parameters.

As you can see, PyUnits is suitable for Serious Scientific Work.


I have a colleague who likes nothing better than copying and pasting code from README files. He refers to this type of file as a "cookbook". Therefore, I will be providing a similar facility in this section.

Building a Unit Library

So far, all the examples in this document have used the example unit library provided in this repository under examples/ That's all well and good, but if you're using PyUnits for real, the first thing you're probably going to want to do is create your own unit library:

from pyunits.unit import StandardUnit, Unit
from pyunits.unit_type import UnitType

class Length(UnitType):
    Type for length units.

class Meters(StandardUnit):
    A meters unit.

    def name(self) -> str:
        See superclass for documentation.
        return "m"

class Centimeters(Unit):
    A centimeters unit.

    def _from_standard(self, standard_value: StandardUnit) -> None:
        See superclass for documentation.
        # Convert from meters.
        self._set_raw(standard_value.raw * 100)

    def to_standard(self) -> Meters:
        See superclass for documentation.
        # Convert to meters.
        return Meters(self.raw / 100)

    def name(self) -> str:
        See superclass for documentation.
        return "cm"

Fundamentally, PyUnits has two types of objects that the user needs to be aware of: Unit and UnitType. The former is easy: a Unit instance simply represents a value with a specific unit. The second is a little harder to explain.

Intuitively, some units can be trivially converted to each-other, eg. Meters and Inches, and some can't be, eg. Meters and Seconds. The UnitType class exists in order to allow PyUnits to understand these relationships. When using PyUnits, every Unit subclass must be decorated with an appropriate UnitType subclass. Two units that are marked with the same UnitType can be trivially converted to each-other. Two units that aren't can't easily be converted, and PyUnits with raise an error if you try to do so.

Standard Units

For all the units of a particular UnitType, PyUnits expects the user to choose a "standard unit". In practice, which unit you use as the standard one doesn't really matter, as long as you can tell PyUnits how to convert from every other unit to the standard one and vice-versa.

In practice, this is done by overriding two methods:

  • The _from_standard() method takes an instance of the standard unit and initializes this Unit instance appropriately with its converted value.
  • The to_standard() method returns an instance of the standard unit with an equivalent value to this one.

Once we have that set up, PyUnits can convert implicitly between all the units of this UnitType:

meters = Meters(10)
centimeters = Centimeters(meters)

print(centimeters)  # prints "1000 cm"

Which unit is considered to be the standard one is defined by which one inherits from the StandardUnit class. (This is Meters in the above example.)


PyUnits has (currently limited) support for pretty-printing unit values. This is utilized by overriding the name property, as seen in the example above. This property should return a suffix that will be appended to the unit value when printing.

Numpy Integration

PyUnits can essentially be thought of as a wrapper around Numpy. That is because Unit subclasses actually store and manipulate Numpy arrays internally:

import numpy as np

from examples import example_units as eu

secs = eu.Seconds(np.array([1, 2, 3]))
print(secs)  # prints "[1 2 3] s"

You can access the raw Numpy value of a unit using the raw property:

print(secs.raw)  # print "[1 2 3]"

Dimensional Analysis

PyUnits is generally clever about unit operations:

from examples import example_units as eu

meters = eu.Meters(2)
seconds = eu.Seconds(4)

print(meters / seconds)  # prints 0.5 m\n-\ns

As can be seen in the earlier examples, it will even go so far as to auto-simplify the results of multiplication and division operations.

Also, compound unit types can be created manually:

from examples import example_units as eu
from pyunits.compound_units import Mul, Div

Watts = Div(eu.Joules, eu.Seconds)
SquareMeters = Mul(eu.Meters, eu.Meters)

# These can then be used like normal units.
watts = Watts(10)
square_meters = SquareMeters(50)

Unitless Values

PyUnits has the special concept of "Unitless" values, which, paradoxically, can be used like a Unit in many cases. These crop up most often when doing division.

from examples import example_units as eu

meters1 = eu.Meters(10)
meters2 = eu.Meters(5)

print(meters1 / meters2)  # prints "2"

The result of this division is an instance of the class Unitless. This class can also be used directly in order to represent concepts such as "inverse meters".

from examples import example_units as eu
from pyunits.compound_units import Div
from pyunits.unitless import Unitless

inverse_meters = Div(Unitless, eu.Meters)

PyUnits will refuse to implicitly convert Unitless instances, however. If you want to use this as a value of a unit, you have to explicitly take the raw value:

from examples import example_units as eu
from pyunits.unitless import Unitless

unitless = Unitless(5)
meters = eu.Meters(unitless)  # Error!

# The correct way...
meters = eu.Meters(unitless.raw)

This is an explicit design choice that was made to avoid cases where values that did not have units could "magically" acquire them.

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