munits (name not capitalized) is a units library for Python. It allows quantities to be stored with their units attached, and simplifies unit conversion somewhat.

Initial Setup

To import the munits library, use the import statement.

>>> import munits

In the examples presented here, the contents of munits are imported directly into the mainspace.

>>> from munits import *

Defining Base Units

To define a base unit, use the BaseUnit class. For this example, we will define pixels as our base unit.

>>> pixel = BaseUnit("px")

This creates a new unit object. To check this, we can print the new object. String representation of BaseUnits is slightly altered in this documentation as the memory locations present in the actual representation change each time the module is loaded. They are omitted here for clarity.

>>> pixel
<BaseUnit px>

munits also defines many standard units in order to improve inter‐operability of code. These are found within the si, si.prefix, us, and data sub‐modules.

>>> si.METRE
<BaseUnit m>

Note that two BaseUnits having the same symbol does not make them compare equal.

>>> spam = BaseUnit("m")
>>> spam == si.METRE
False
>>> del spam  # cleaning up this demonstration

Using Units

To add units to a quantity, multiply the value with the units to be applied.

>>> 5 * pixel
5.0 * <BaseUnit px>
>>> 7.4 * si.METRE
7.4 * <BaseUnit m>

Quantities can take multiple units, as well as units with exponents.

>>> 25 * si.METRE / si.SECOND
25.0 * <BaseUnit m> / <BaseUnit s>
>>> 13 * si.METRE**2
13.0 * <BaseUnit m>**2.0
>>> (13 * si.METRE)**2
169.0 * <BaseUnit m>**2.0

Derived Units

Unlike the base units described above, derived units are defined in terms of other units. They can be defined using the DerivedUnit class.

>>> megapixel = DerivedUnit("Mpx", 1e6 * pixel)

As described above, munits comes with many standard units, many of which are derived from other included units.

>>> si.prefix.KILOMETER
DerivedUnit("km", 1000.0 * <BaseUnit m>)
>>> si.NEWTON
DerivedUnit("N", 1.0 * <BaseUnit kg> * <BaseUnit m> * <BaseUnit s>**-2.0)

It is also possible to create new DerivedUnits using built‐in BaseUnits.

>>> frame = DerivedUnit("f", si.SECOND / 60)

Other Classes

UnitQuantity

A UnitQuantity represents a quantity that has units. This class supports many operations. UnitQuantitys can be added and subtracted with other UnitQuantitys that have compatible units. The compatible method can be used to test if two UnitQuantitys have compatible units. The convert method is used to convert between compatible units. Equality checks can be performed with any pair of UnitQuantitys, and comparisons (<, >, <=, or >=) can be performed with UnitQuantitys with compatible units. UnitQuantitys with units that cancel can be converted to floats and floats can be converted to UnitQuantitys with blank units. Other mathematical operations including multiplication and exponentiation. The normalize method converts a UnitQuantity into another of the same value with only BaseUnits. The round method (different to build‐in round!) can be used to round UnitQuantitys, with a customizable rounding interval (for example, round to the nearest 5km) and rounding mode.

UnitCombination

A subclass of UnitQuantity with a fixed value of 1. The main purpose is to specify a unit type without specifying a value, such as in UnitQuantity.convert. The units property of UnitQuantity objects also returns a value of this type.

Unit

An abstract class representing a generic unit.