quantity 0.7.3

Unit-safe computations with quantities

The package quantity provides classes for unit-safe computations with quantities.

Defining a quantity class

A basic type of quantity is declared just by sub-classing the class Quantity:

>>> class Length(Quantity):
...     pass

In addition to the new quantity class the meta-class of class Quantity creates a corresponding class for the units automatically. It can be referenced via the quantity class:

>>> Length.Unit
<class 'quantity.quantity.LengthUnit'>

If the quantity has a unit which is used as a reference for defining other units, the simplest way to define it is giving a name and a symbol for it as class variables. The meta-class of class Quantity will then create a unit automatically:

>>> class Length(Quantity):
...     refUnitName = 'Meter'
...     refUnitSymbol = 'm'
>>> Length.refUnit
Length.Unit('m')

Now, this unit can be given to create a quantity:

>>> METER = Length.refUnit
>>> print(Length(15, METER))
15 m

If no unit is given, the reference unit is used:

>>> print(Length(15))
15 m

Other units can be derived from the reference unit (or another unit), giving a definition by multiplying a scaling factor with that unit:

>>> MILLIMETER = Length.Unit('mm', 'Millimeter', Decimal('0.001') * METER)
>>> MILLIMETER
Length.Unit('mm')
>>> KILOMETER = Length.Unit('km', 'Kilometer', 1000 * METER)
>>> KILOMETER
Length.Unit('km')
>>> CENTIMETER = Length.Unit('cm', 'Centimeter', 10 * MILLIMETER)
>>> CENTIMETER
Length.Unit('cm')

Using one unit as a reference and defining all other units by giving a scaling factor is only possible if the units have the same scale. Otherwise, units have to be instantiated via the coresponding class Unit sub-class without giving a definition.

>>> class Temperature(Quantity):
...     pass
>>> CELSIUS = Temperature.Unit('°C', 'Degree Celsius')
>>> FAHRENHEIT = Temperature.Unit('°F', 'Degree Fahrenheit')

Derived types of quantities are declared by giving a definition based on more basic types of quantities:

>>> class Volume(Quantity):
...     defineAs = Length ** 3
...     refUnitName = 'Cubic Meter'
>>> class Duration(Quantity):
...     refUnitName = 'Second'
...     refUnitSymbol = 's'
>>> class Velocity(Quantity):
...     defineAs = Length / Duration
...     refUnitName = 'Meter per Second'

If no symbol for the reference unit is given with the class declaration, a symbol is generated from the definition, as long as all types of quantities in that definition have a reference unit.

>>> print(Volume.refUnit.symbol)
m³
>>> print(Velocity.refUnit.symbol)
m/s

Instantiating quantities

The simplest way to create an instance of a class Quantity subclass is to call the class giving an amount and a unit. If the unit is omitted, the quantity’s reference unit is used (if one is defined).

>>> Length(15, MILLIMETER)
Length(Decimal(15), Length.Unit(u'mm'))

Alternatively, the two-args infix operator ‘^’ can be used to combine an amount and a unit:

>>> 17.5 ^ KILOMETER
Length(Decimal('17.5'), Length.Unit(u'km'))

Also, it’s possible to create a Quantity sub-class instance from a string representation:

>>> Length('17.5 km')
Length(Decimal('17.5'), Length.Unit(u'km'))

Unit-safe computations

A quantity can be converted to a quantity using a different unit by calling the method Quantity.convert:

>>> l5cm = Length(Decimal(5), CENTIMETER)
>>> l5cm.convert(MILLIMETER)
Length(Decimal('50'), Length.Unit('mm'))
>>> l5cm.convert(KILOMETER)
Length(Decimal('0.00005'), Length.Unit('km'))

Quantities can be compared to other quantities using all comparison operators defined for numbers. Different units are taken in to account automatically, as long as they are compatible, i. e. a conversion is available:

>>> Length(27) <= Length(91)
True
>>> Length(27, METER) <= Length(91, CENTIMETER)
False

Quantities can be added to or subtracted from other quantities …

>>> Length(27) + Length(9)
Length(Decimal(36))
>>> Length(27) - Length(91)
Length(Decimal(-64))
>>> Length(27) + Length(12, CENTIMETER)
Length(Decimal('27.12'))
>>> Length(12, CENTIMETER) + Length(17, METER)
Length(Decimal('1712'), Length.Unit('cm'))

… as long as they are instances of the same quantity type:

>>> Length(27) + Duration(9)
quantity.quantity.IncompatibleUnitsError: Can't add a 'Length' and a
    'Duration'

Quantities can be multiplied or divided by scalars, preserving the unit:

>>> 7.5 * Length(3, CENTIMETER)
Length(Decimal('22.5'), Length.Unit(u'cm'))
>>> Duration(66, MINUTE) / 11
Duration(Decimal(6), Duration.Unit(u'min'))

Quantities can be multiplied or divided by other quantities …:

>>> Length(15, METER) / Duration(3, SECOND)
Velocity(Decimal(5))

… as long as the resulting type of quantity is defined …:

>>> Duration(4, SECOND) * Length(7)
UndefinedResultError: Undefined result: Duration * Length

… or the result is a scalar:

>>> Duration(2, MINUTE) / Duration(50, SECOND)
Decimal('2.4')

For more details see the documentation provided with the source distribution or here.

History

Version	Changes
0.7.3	Added function ‘sum’. Added method ‘Quantity.allocate’.
0.7.2	Prevent __dict__ from being built for subclasses of Quantity or Unit. Made ‘refUnitSymbol’ and ‘refUnitName’ read-only properties. Added class property ‘quantum’ as option to define quantized quantities. Removed ‘__version__’ from all sub-modules.
0.7.1	Added ‘install_requires’ to setup.py. Corrected unit of ‘Kelvin’ in doc and README. Added method ‘Quantity.quantize’. Enhanced method ‘Quantity.__round__’: round to quantity or unit.
0.7.0	First public release.