# Creating your own Measure Class

You can create your own measures easily by subclassing either `measurement.base.MeasureBase` or `measurement.base.BidimensionalMeasure`.

## Simple Measures

If your measure is not a measure dependent upon another measure (e.g speed, distance/time) you can create new measurement by creating a subclass of `measurement.base.MeasureBase`.

A simple example is Weight:

    from measurement.base import MeasureBase

    class Weight(MeasureBase):
        STANDARD_UNIT = 'g'
        UNITS = {
            'g': 1.0,
            'tonne': 1000000.0,
            'oz': 28.3495,
            'lb': 453.592,
            'stone': 6350.29,
            'short_ton': 907185.0,
            'long_ton': 1016000.0,
        }
        ALIAS = {
            'gram': 'g',
            'ton': 'short_ton',
            'metric tonne': 'tonne',
            'metric ton': 'tonne',
            'ounce': 'oz',
            'pound': 'lb',
            'short ton': 'short_ton',
            'long ton': 'long_ton',
        }
        SI_UNITS = ['g']

Important details:

- `STANDARD_UNIT` defines what unit will be used internally by the library for storing the value of this measurement.

- `UNITS` provides a mapping relating a unit of your `STANDARD_UNIT` to any number of defined units. In the example above, you will see that we’ve established `28.3495 g` to be equal to `1 oz`.

- `ALIAS` provides a list of aliases mapping keyword arguments to `UNITS`. these values are allowed to be used as keyword arguments when either creating a new unit or guessing a measurement using `measurement.utils.guess`.

- `SI_UNITS` provides a list of units that are SI Units. Units in this list will automatically have new units and aliases created for each of the main SI magnitudes. In the above example, this causes the list of `UNITS` and `ALIAS` es to be extended to include the following units (aliases): `yg` (yottagrams), `zg` (zeptograms), `ag` (attograms), `fg` (femtograms), `pg` (picograms), `ng` (nanograms), `ug` (micrograms), `mg` (milligrams), `kg` (kilograms), `Mg` (megagrams), `Gg` (gigagrams), `Tg` (teragrams), `Pg` (petagrams), `Eg` (exagrams), `Zg` (zetagrams), `Yg` (yottagrams).

### Using formula-based conversions

In some situations, your conversions between units may not be simple enough to be accomplished by using simple conversions (e.g. temperature); for situations like that, you should use `sympy` to create expressions relating your measure’s standard unit and the unit you’re defining:

    from sympy import S, Symbol
    from measurement.base import MeasureBase

    class Temperature(MeasureBase):
        SU = Symbol('kelvin')
        STANDARD_UNIT = 'k'
        UNITS = {
            'c': SU - S(273.15),
            'f': (SU - S(273.15)) * S('9/5') + 32,
            'k': 1.0
        }
        ALIAS = {
            'celsius': 'c',
            'fahrenheit': 'f',
            'kelvin': 'k',
        }

Important details:

- See above ‘Important Details’ under Normal Measures.

- `SU` must define the symbol used in expressions relating your measure’s `STANDARD_UNIT` to the unit you’re defining.

## Bi-dimensional Measures

Some measures are really just compositions of two separate measures – Speed, being a measure of the amount of distance covered over a unit of time, is one common example of such a measure.

You can create such measures by subclassing `measurement.base.BidimensionalMeasure`.

    from measurement.base import BidimensionalMeasure

    from measurement.measures.distance import Distance
    from measurement.measures.time import Time

    class Speed(BidimensionalMeasure):
        PRIMARY_DIMENSION = Distance
        REFERENCE_DIMENSION = Time

        ALIAS = {
            'mph': 'mi__hr',
            'kph': 'km__hr',
        }

Important details:

- `PRIMARY_DIMENSION` is a class that measures the variable dimension of this measure. In the case of ‘miles-per-hour’, this would be the ‘miles’ or ‘distance’ dimension of the measurement.

- `REFERENCE_DIMENSION` is a class that measures the unit (reference) dimension of the measure. In the case of ‘miles-per-hour’, this would be the ‘hour’ or ‘time’ dimension of the measurement.

- `ALIAS` defines a list of convenient abbreviations for use either when creating or defining a new instance of this measurement. In the above case, you can create an instance of speed like `Speed(mph=10)` (equivalent to `Speed(mile__hour=10)`) or convert to an existing measurement ( `speed_measurement`) into one of the aliased measures by accessing the attribute named – `speed_measurement.kph` (equivalent to `speed_measurement.kilometer__hour`).

Note

Although unit aliases defined in a bi-dimensional measurement’s `ALIAS` dictionary can be used either as keyword arguments or as attributes used for conversion, unit aliases defined in simple measurements (those subclassing `measurement.base.MeasureBase`) can be used only as keyword arguments.