misu 1.0.4

Fast quantities

misu

misu is short for “misura”, which means measurement (in Italian).

Demo

Most of the time you will probably work with misu interactively, and it will be most convenient to import the entire namespace:

from misu import *

mass = 100*kg
print(mass >> lb)

The symbol kg got imported from the misu package. We redefine the shift operator to perform inline conversions. The code above produces:

220.46226218487757

There are many units already defined, and it is easy to add more. Here we convert the same quantity into ounces:

print(mass >> oz)

output:

3571.4285714285716

What you see above would be useless on its own. What you really need is to be able to perform consistent calculations with quantities expressed in different, but compatible units:

mass = 10*kg + 20*lb
print(mass)

output:

19.07 kg

For addition and subtraction, misu will ensure that only consistent units can be used. Multiplication and division will produce new units:

distance = 100*metres
time = 9.2*seconds
​
speed = distance / time
print(speed)

output:

10.87 m/s

As before, it is trivially easy to express that quantity in different units of compatible dimensions:

print(speed >> km/hr)

output:

39.130434782608695

Introduction

misu is a package of handling physical quantities with dimensions. This means performing calculations with all the units being tracked correctly. It is possible to add kilograms per hour to ounces per minute, obtain the correct answer, and have that answer be reported in, say, pounds per week.

misu grew out of a personal need. I have used this code personally in a (chemical) engineering context for well over a year now (at time of writing, Feb 2015). Every feature has been added in response to a personal need.

Features

• Speed optimized. misu is very fast! Heavy math code in Python will be around only 5X slower when used with misu. This is much faster than other quantities packages for Python.
• Written as a Cython extension module. Speed benefits carry over when using misu from your own Cython module (a .pxd is provided for linking).
• When an operation involving incompatible units is attempted, an EIncompatibleUnits exception is raised, with a clear explanation message about which units were inconsistent.
• Decorators for functions to enforce dimensions

@dimensions(x='Length', y='Mass')
def f(x, y):
    return x/y

f(2*m, 3*kg)         # Works
f(200*feet, 3*tons)  # Works

f(2*joules, 3*kelvin)  # raises AssertionError
f(2*m, 3)              # raises AssertionError

• An operator for easily stripping the units component to obtain a plain numerical value

mass = 100 * kg
mass_lb = mass >> lb

duty = 50 * MW
duty_BTU_hr = duty >> BTU / hr

• An enormous amount of redundancy in the naming of various units. This means that m, metre, metres, METRE, METRES will all work. The reason for this is that from my own experience, when working interactively (e.g. in the IPython Notebook) it can be very distracting to incorrectly guess the name for a particular unit, and have to look it up. ft, foot and feet all work, m3 means m**3 and so on.
• You can specify a reporting unit for a dimension, meaning that you could have all lengths be reported in “feet” by default for example.
• You can specify a reporting format for a particular unit.

There are other projects, why misu?

There are several units systems for Python, but the primary motivating use-case is that misu is written as a Cython module and is by far the fastest* for managing units available in Python.

*Except for ``NumericalUnits``, which is a special case

**I haven’t actually checked that this statement is true for all of them yet.

General usage

For speed-critical code, the application of unit operations can still be too slow. In these situations it is typical to first cast quantities into numerical values (doubles, say), perform the speed-critical calculations (perhaps call into a C-library), and then re-cast the result back into a quantity and return that from a function.

@dimensions(x='Length', y='Mass')
def f(x, y):
    x = x >> metre
    y = y >> ounces
    <code that assumes meters and ounces, returns value in BTU>
    return answer * BTU

This way you can still easily wrap performance-critical calculations with robust unit-handling.

Inspiration

The inspiration for misu was Frink by Alan Eliasen. It is wonderful, but I need to work with units in the IPython Notebook, and with all my other Python code.

There are a bunch of other similar projects. I have not used any of them enough yet to provide a fair comparison:

• astropy.units
• Buckingham
• DimPy
• Magnitude
• NumericalUnits
• Pint
• Python-quantities
• Scalar
• Scientific.Physics.PhysicalQuantities
• SciMath
• sympy.physics.units
• udunitspy
• Units
• Unum