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 STANDRD_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 Tempoerature(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.