Quantity¶
The Quantity object is meant to represent a value that has some unit associated with the number.
Creating Quantity instances¶
Quantity objects are created through multiplication or divison with
Unit objects. For example, to create a Quantity to represent
>>> import astropy.units as u
>>> 15*u.m/u.s
<Quantity 15 m / (s)>
or 
>>> 1.14/u.s
<Quantity 1.14 1 / (s)>
You can also create instances using the Quantity constructor directly, by specifying a value and unit
>>> u.Quantity(15, u.m/u.s)
<Quantity 15 m / (s)>
Arithmetic¶
Addition and Subtraction¶
Addition or subtraction between Quantity objects is supported when their units are equivalent (see Unit documentation). When the units are equal, the resulting object has the same unit
>>> 11*u.s + 30*u.s
<Quantity 41 s>
>>> 30*u.s - 11*u.s
<Quantity 19 s>
If the units are equivalent, but not equal (e.g. kilometer and meter), the resulting object has units of the object on the left
>>> 1100.1*u.m + 13.5*u.km
<Quantity 14600.1 m>
>>> 13.5*u.km + 1100.1*u.m
<Quantity 14.6001 km>
>>> 1100.1*u.m - 13.5*u.km
<Quantity -12399.9 m>
>>> 13.5*u.km - 1100.1*u.m
<Quantity 12.3999 km>
Addition and subtraction is not supported between Quantity objects and basic numeric types
>>> 13.5*u.km + 19.412
TypeError: Object of type '<type 'float'>' cannot be added with a Quantity object. Addition is only supported between Quantity objects.
Multiplication and Division¶
Multiplication and division is supported between Quantity objects with any units, and with numeric types. For these operations between objects with equivalent units, the resulting object has composite units
>>> 1.1*u.m * 140.3*u.cm
<Quantity 154.33 cm m>
>>> 140.3*u.cm * 1.1*u.m
<Quantity 154.33 cm m>
>>> 1.*u.m / (20.*u.cm)
<Quantity 0.05 m / (cm)>
>>> 20.*u.cm / (1.*u.m)
<Quantity 20.0 cm / (m)>
For multiplication, you can choose how to represent the resulting object by using the to method
>>> (1.1*u.m * 140.3*u.cm).to(u.m**2)
<Quantity 1.5433 m2>
>>> (1.1*u.m * 140.3*u.cm).to(u.cm**2)
<Quantity 15433.0 cm2>
For division, if the units are equivalent, you may want to make the resulting object dimensionless by reducing the units. To do this, use the simplify_units() method
>>> (20.*u.cm / (1.*u.m)).simplify_units()
<Quantity 0.2 >
This method is also useful for more complicated arithmetic
>>> 15.*u.kg * 32.*u.cm * 15*u.m / (11.*u.s * 1914.15*u.ms)
<Quantity 0.341950972779 cm kg m / (ms s)>
>>> (15.*u.kg * 32.*u.cm * 15*u.m / (11.*u.s * 1914.15*u.ms)).simplify_units()
<Quantity 3.41950972779 kg m2 / (s2)>