Quantities arrays are designed to work like normal numpy arrays. However, a few operations are not yet fully functioning.
In the following code examples, it’s assumed that you’ve initiated the following imports:
>>> import numpy as np >>> import quantities as pq
Quantities is not designed to handle coordinate systems that require a point of reference, like positions on a map or absolute temperature scales. Proper support of coordinate systems would be a fairly large undertaking and is outside the scope of this project. Furthermore, consider the following:
>>> T_0 = 100 * pq.K >>> T_1 = 200 * pq.K >>> dT = T_1-T_0 >>> dT.units = pq.degF
To properly support the above example, quantities would have to distinguish absolute temperatures with temperature differences. It would have to know how to combine these two different animals, etc. The quantities project has therefore elected to limit the scope to relative quantities.
As a consequence, quantities treats temperatures as a temperature difference. This is a distinction without a difference when considering Kelvin and Rankine, or transformations between the two scales, since both scales have zero offset. Temperature scales in Celsius and Fahrenheit are different and would require a non-zero offset, which is not supported in Quantities unit transformation framework.
Many common math functions ignore the dimensions of quantities. For example, trigonometric functions (e.g. np.sin) suffer this fate. For these functions, quantities arrays are treated like normal arrays and the calculations proceed as normal (except that a “not implemented” warning is raised). Note, however, this behavior is not ideal since some functions should behave differently for different units. For example, you would expect np.sin to give different results for an angle of 1° versus an angle of 1 radian; instead, np.sin extracts the magnitude of the input and assumes that it is already in radians.
To properly handle quantities, use the corresponding quantities functions whenever possible. For example, pq.sin will properly handle the angle inputs described above. For an exhaustive list, see the functions defined in pq.umath.
Functions which ignore/drop units¶
There are additional numpy functions not in pq.umath that ignore and drop units. Below is a list known functions in this category