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Python Units of Measure

Introduction

This is a Python library for representing and converting physical units of measure. It is a port of the php library by Jonathan Hanson, found here:

php-units-of-measure.git

The utility of this library is in encapsulating physical quantities in such a way that you don't have to keep track of which unit they're represented in. For instance:

from pyuom.Length import Length

height = Length(6.16, 'feet')
print height.toUnit('m')

# would print 1.87757, which is 6.16 feet in meters.

Having this abstraction allows you to create interfaces that accept physical quantities without requiring them to be in a particular unit. For example, this function assumes the height is a float of a particular unit (presumably feet):

def isTooTallToRideThisTrain(height):
    return height > 5


// Calling the function requires that you first convert whatever quantity you have into the expected units:
```python
isTooTallToRideThisTrain(2 / 0.3048)

Whereas this version allows for height to be provided in whatever unit is convenient:

from Length import Length

def isTooTallToRideThisTrain( height )
    return height.toUnit('ft') > 5

# Calling the function now allows any unit to be used:

isTooTallToRideThisTrain( Length(2, 'm') )

Installation

Use the binary for Windows, or unzip the zipfile into your site-packages directory.

Use

Conversion

As in the examples above, the basic usage of this library is in representing physical quantities and converting between typical units of measure. For example:

from pyuom.Mass import Mass

quantity = Mass(6, 'lbs')
print quantity.toUnit('g')

It's also possible to implicity cast a quantity to a string, which will display its original value:

quantity = Mass(6, 'pounds')
print quantity # '6 lbs'

Arithmetic Operators

There's also support for addition and subtraction. The PhysicalQuantity objects are immutable, and as such these arithmetic methods return new quantity objects representing the results:

from pyuom.Volume import Volume

first  = Volume(6, 'liters')
second = Volume(6, 'cups')

sum = first.add(second)
print sum # 7.4195292 l

difference = first.subtract(second)
print difference # 4.5804708 l

Adding new Units of Measure to Existing Quantities

Ocassionally, you will need to add a new unit of measure to a pre-existing quantity.

For example, let's say in a project you need a new measure of length, called "cubits". You have two options: you can permanently add the new unit of measure to the \pyuom\Length class (and submit a pull request to get it added upstream, if appropriate), or you can add the unit temporarily at run time, inside your calling code.

Adding a New Unit of Measure at Runtime

To add a new unit of measure to an existing quantity at run time, you'd do this:

from pyuom.Length import Length
from pyuom.UnitOfMeasure import UnitOfMeasure

# It's ok to use cubits here, since the conversion doesn't happen until later
length = Length(14, 'cubits')

# Build a new Unit of Measure object which represents the new unit, and which knows how to convert between
# the new unit and the quantity's native unit (in this case, meters).

cubit = UnitOfMeasure(
    # This is the official name of this unit - typically it's the standard abbreviation
    'cb',

    # The second parameter is a closure that converts from the native unit to this unit
    lambda x: x / 0.4572,

    # The third parameter is a closure that converts from this unit to the native unit
    lambda y: y * 0.4572
    )

# Any alias names for this unit can be added here, to make it easier to use variations
cubit.addAlias('cubit')
cubit.addAlias('cubits')

# Register the new unit of measure with the quantity object
length.registerUnitOfMeasure(cubit)

# Now that the unit is registered, you can cast the measurement to any other measure of length
print length.toUnit('feet') # '21'

Permanently Adding a New Unit of Measure to a Physical Quantity

The above method only applies to the specific Length object and is therefore temporary; it would be necessary to repeat this process every time you created a new measurement and wanted to use cubits.

A new unit of measure can be permanently added to a physical quantity class by essentially the same process, only it would be done inside the constructor of the quantity class.

Adding New Physical Quantities

Physical quantities are categories of measurable values, like mass, length, force, etc.

For physical quantities that are not already present in this library, it will be necessary to write a class to support a new one. All physical quantities extend the \pyuom\PhysicalQuantity class, and typically have only a constructor method which creates the quantity's units of measure. See above for examples on how to add new units to a quantity class.

Note that every physical quantity has a chosen "native unit" which is typically the SI standard. The main point for this unit is that all of the quantity's other units of measure will convert to and from this chosen native unit. It's important to be aware of a quantity's native unit when writing conversions for new units of measure.

Adding new Aliases to Existing Units

It may come up that the right unit of measure exists for the right physical quantity, but there's a missing alias for the unit. For example, if you thought 'footses' was an obviously lacking alias for the Length unit 'ft', you could temporarily add the alias like this:

from pyuom.Length import Length

# It's ok to use footses here, since the conversion doesn't happen until later
length = Length(4, 'footses')

# Fetch the unit of measure object that represents the 'ft' unit
foot = length.findUnitDefinition('ft')

# Any alias names for this unit can be added here, to make it easier to use variations
foot.addAlias('footses')

# Now that the unit is registered, you can cast the measurement to any other measure of length
print length.toUnit('m') # '1.2192'

And of course, if you need to add the alias permanently, you can do so in the constructor of the quantity's class.

Testing and Contributing

Pull requests are welcome, especially regarding new units of measure or new physical quantities. However, please note that there are many sources for conversion factors, and not all are careful to respect known precision.

In the United States, the standards body for measurement is NIST, and they've published NIST Special Publication 1038 "The International System of Units (SI) - Conversion factors for General Use". This guide contains the approved conversion factors between various units and the base SI units.

Also note that any new physical quantities should have the appropriate SI unit chosen for their native unit of measure.