SymPy: How to return an expression in terms of other [removed]s)?

Question

I\'m fairly new to SymPy and have what might be a basic question. Or I might simply be misinterpreting how SymPy is supposed to be used.

Is there a way to create an

Answer 1

In sympy units you convert from expr1 to expr2 by dividing:

In [120]: import sympy.physics.units as units

In [121]: expr1 = units.m / units.s

In [122]: expr2 = units.miles / units.hour

In [123]: a = (1397/3125) * expr1 / expr2

In [124]: a
Out[124]: 1

The only issue is that you end up with a dimensionless quantity. You might also consider the Quantities package for this type of thing:

In [125]: import quantities as pq

In [126]: a = pq.Quantity(1397/3125, 'm/s')

In [127]: a
Out[127]: array(0.44703999999999999) * m/s

In [128]: a.units = pq.mile / pq.hour

In [129]: a
Out[129]: array(1.0) * mi/h

There's also Unum.

I don't know of a PDF version of the sympy docs, but you might be able to check the distribution out from their repository and use Sphinx to generate a PDF for yourself.

Answer 2

there's also quantities:

In [1]: from quantities import *
In [2]: v = 1397*m / (3125*s)
In [3]: v
Out[3]: array(0.44704) * m/s
In [8]: v.units = mile/hour
In [9]: v
Out[9]: array(1.0) * mi/h

Answer 3

Checking the source for sympy.physics.units you can see that all units are defined in terms of meters, kilograms, seconds, amperes, kelvins, moles and candelas. These are the base units.

Then a mile is defined as 5280 feet, and a foot is defined as 0.3048 meters.

So all expressions using non-base units will have the non-base units replaced with the base units.

You can define your own units which you can subsitute into an expression when you need an expression to use particular units:

import sympy.physics.units as units
from sympy import Rational

my_mile = units.Unit('my_mile', 'mile')
my_hour = units.Unit('my_hour', 'hour')

Then define a dictionary which will substitute the base units for your new units.

converter = {units.m: my_mile/Rational('1609.344'),
             units.s: my_hour/Rational('3600')}

Perform all your calculations using the base units. Then if you want a value using miles and hours, you can substitute your new units into the expression.

v = 10*units.miles/units.hour
print v # = 2794*m/(625*s)

print v.subs(converter) # = 10*mile/hour

Use ars answer for getting the docs. Sympy's repository is here: https://github.com/sympy/sympy

There is a README file in the docs folder which describes how to create html docs.