Displaying quantities in SI base units

[Teo]

Hi all. I just started using Sympy this week. According to this video, when the following code was run, the units of ohm would be returned in SI base units.

import sympy.physics.units as u

u.ohm

However, when I tried, I got the symbol omega instead. Is there a method to obtain the same SI base units (kgm^2)/(A^2s^3)?

Thank you

[Oscar Benjamin]

You can do it like this:

In [1]: import sympy.physics.units as u

In [3]: u.convert_to(u.ohm, [u.kg,u.m,u.ampere,u.s])
Out[3]:
2
kilogram⋅meter
───────────────
2 3
ampere ⋅second

Probably there should be an easier way. It's also possible like this:

In [4]: u.convert_to(u.ohm, u.si.SI._base_units)
Out[4]:
2
kilogram⋅meter
───────────────
2 3
ampere ⋅second

In [5]: u.si.SI._base_units
Out[5]: (meter, kilogram, second, ampere, mole, candela, kelvin)

That's using _base_units which has a leading underscore indicating
that it should be considered "private". I don't see why it should be
considered private though...

Oscar

[Teo]

Thanks Oscar, that works. I can see why the current output is preferred by many, but the SI base units output is still useful for learners of Physics. My learners are young, so I hope there could be an easier way to achieve this.

[gu...@uwosh.edu]

Teo,

My solution is to treat units as positive valued symbols and skip the units tool in sympy. Then you just define things in the set of units you wish to use and force your students to think about converting among them. For some examples see the documentation for Algebra_with_Sympy. I then use the .subs() to substitute in conversion factors.

Jonathan

[Chris Smith]

I referenced this discussion in https://github.com/sympy/sympy/issues/23022

/c

[Aaron Meurer]

On Sat, Nov 26, 2022 at 12:13 AM Teo <pss....@gmail.com> wrote:
>
> Hi all. I just started using Sympy this week. According to this video, when the following code was run, the units of ohm would be returned in SI base units.

I didn't know about that video. I'm always happy to see more video
tutorial content for SymPy, and community created content in general.

>
> import sympy.physics.units as u
> u.ohm
>
> However, when I tried, I got the symbol omega instead. Is there a method to obtain the same SI base units (kgm^2)/(A^2s^3)?

The units module was rewritten since this video. You can see
https://docs.sympy.org/latest/modules/physics/units/index.html for a
description of the new module. The new module is designed to be
general enough to handle different unit systems, not just SI, which is
why it no longer automatically converts everything down to SI base
units.

Aaron Meurer

>
> Thank you
>
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