CODEWITHSUNDEEP
sympy
lyschoening
lyschoening

Reputation: 18738

How to simplify units with different scales in SymPy

In sympy.physics.units identical units automatically simplify when they are used in equations. However, when units are different scales of the same dimension, they are not automatically simplified. For example:

from sympy.physics.units import *

(5000 * gram) / (2 * gram)
# 2500

(5 * kilogram) / (2 * gram)
# (5 * kilogram) / (2 * gram)

5 * gram + 1 * kilogram
# 5*gram + kilogram

5 * gram + 1000 * gram
# 1005*gram

1000*liter/(5000*liter)
# 1/5

1000*liter/(5*meter**3)
# 200*liter/meter**3

Is there an easy way to force unit simplification, at least in cases where the units are different scales of the same dimension?

I am looking for the same effect as using .subs() on all the units, just automated to use the information already contained in the expression, equivalent to

((5 * kilogram) / (2 * gram)).subs({gram: gram, kilogram: 1000 * gram, ...})

Upvotes: 3

Views: 617

Answers (1)

user6655984
user6655984

Reputation:

The issue of scale can be solved by print_unit_base method of the unit system. For example,

from sympy.physics.units.systems import MKS
expr = (5 * kilogram) / (2 * gram)
new_expr = expr.xreplace({q: MKS.print_unit_base(q) for q in expr.atoms(Quantity)})

Now new_expr is 2500.

A simpler function to use is convert_to(expr, kilogram) (which also returns 2500) but this depends on knowing that units of mass are an issue. print_unit_base uniformizes all units of the given unit system.

With liter, since it's not really a prefixed-meter-cubed, we need convert_to:

expr2 = 1000*liter/(5*meter**3)
convert_to(expr2, meter)     #  1/5

Upvotes: 2

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