Units

Units

Value-bearing attributes are first-class in sysml2py. Pass a value and a unit and the library renders the SysML [unit] syntax, importing the unit automatically.

Basic values

from sysml2py import Attribute

Attribute('mass', value=1000, unit='kg').dump()
'attribute mass = 1000[kg];'

When the attribute lives in a container, the unit is imported as a public import (SysML v2 / KERML-74 requires an explicit visibility modifier — bare import is not valid):

pkg = PackageBuilder('Rocket')
stage = Part('Stage_1')
stage.add(Attribute('mass', value=1000, unit='kg'))
pkg.add(stage)
print(pkg.dump())

package Rocket
{
   public import SI::kg;
   part Stage_1
   {
      attribute mass = 1000[kg];
   }
}

Using quantity objects (pint or astropy)

If you prefer, pass a pint or astropy Quantity and the unit is derived from it — either works as a drop-in convenience:

# pint
import pint
ureg = pint.UnitRegistry()
Attribute('mass', value=1000 * ureg.kg)   # attribute mass = 1000[kg];

# astropy (the original Quantity integration, still supported)
import astropy.units as u
Attribute('mass', value=1000 * u.kg)      # attribute mass = 1000[kg];

Both render composite units faithfully, e.g. 9.8 * ureg.m / ureg.s**2 → attribute accel = 9.8[m/s**2];, and both import the unit as a public import in the right namespace (SI base units → SI::, imperial → USCustomaryUnits::).

The sysml2py runtime itself is pure standard library — pint/astropy are not bundled. If you use a Quantity form, add the import and a pint/astropy dependency to your own project.

Composite units

Systems like m/s**2 and / denominators are supported by the generator's unit handling (see the sysml2py-lab corpus tests and the units spec).