Using SageTeX with siunitx

Question

I have the following problem: I'd like to do calculations using SageTeX and use the result in an siunitx expression, so that (for example)

\SI{\sage{3.5^2}}{kg m}

would give me the same result as

\SI{12.25}{kg m}

However, due to how SageTeX works, it's not as easy as this. Trying to compile the following document:

\documentclass{scrartcl}

\usepackage{siunitx}
\usepackage{sagetex}

\begin{document}

\SI{\sage{3.5^2}}{kg m}

\end{document}

Gives me this:

pdflatex -interaction=nonstopmode test.tex

This is pdfTeX, Version 3.1415926-2.5-1.40.14 (TeX Live 2013/Debian)
 restricted \write18 enabled.
entering extended mode
(./test.tex
LaTeX2e <2011/06/27>
Babel <3.9h> and hyphenation patterns for 9 languages loaded.

...

No file test.sagetex.sout.
)
No file test.aux.

...

! Argument of \reserved@a has an extra }.
<inserted text> 
                \par 
l.8 \SI{\sage{3.5^2}}{kg m}

Runaway argument?
{\def \@currenvir {NoHyper}\edef \@currenvline {\on@line }\csname NoH\ETC.
! Paragraph ended before \reserved@a was complete.
<to be read again> 
                   \par 
l.8 \SI{\sage{3.5^2}}{kg m}


!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! siunitx error: "invalid-token-in-number"
! 
! Invalid token '\immediate ' in numerical input.
! 
! See the siunitx documentation for further information.
! 
! For immediate help type H <return>.
!...............................................  

l.8 \SI{\sage{3.5^2}}{kg m}

! Argument of \reserved@a has an extra }.
<inserted text> 
                \par 
l.8 \SI{\sage{3.5^2}}{kg m}

Runaway argument?
{\def \@currenvir {NoHyper}\edef \@currenvline {\on@line }\csname NoH\ETC.
! Paragraph ended before \reserved@a was complete.
<to be read again> 
                   \par 
l.8 \SI{\sage{3.5^2}}{kg m}


!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! siunitx error: "invalid-number"
! 
! Invalid numerical input '\immediate \write \ST@sf {try:
 _st_.inline(0,
! latex(3.5^2))
except:
 _st_.goboom(8)}\def \par '.
! 
! See the siunitx documentation for further information.
! 
! For immediate help type H <return>.
!...............................................  

l.8 \SI{\sage{3.5^2}}{kg m}

*********************************************************************
[1{/var/lib/texmf/fonts/map/pdftex/updmap/pdftex.map}] (./test.aux) )
(\end occurred when \iffalse on line 8 was incomplete)
(\end occurred when \iffalse on line 8 was incomplete)
(see the transcript file for additional information)</usr/share/texlive/texmf-d
ist/fonts/type1/public/amsfonts/cm/cmr10.pfb>
Output written on test.pdf (1 page, 8198 bytes).
Transcript written on test.log.

Unfortunately, I don't understand (La)TeX's internals well enough to decide whether there is a feasible solution to this. Is it possible to work around this somehow in LaTeX, or would it be better to try to program something in Sage itself?

Answer 1

Everyone can get access to Sage with Sagemath Cloud; no installation necessay. I've found that the way around the problems of not working when combined with other packages is to do the calculations in sagesilent mode and then create the text that gets inserted into your document (see for example, my answer here). I still found an issue with the output giving me too many digits, even when I specified how many I wanted. Converting it to a string gave me control over the format. I'm no expert in sagetex so maybe there's a better way, but the code works.

\documentclass{scrartcl}
\usepackage{siunitx}
\usepackage{sagetex}
\begin{document}
\begin{sagesilent}
answer = 4.5^2
output1 = r"\SI{%f}{kg m}"%(answer.n(digits=4))
output2 = r"\SI{%s}{kg m}"%(str(answer.n(digits=4)))
\end{sagesilent}
Output without using a string:
\sagestr{output1}

Output using a string:
\sagestr{output2}
\end{document} 

Here's the code running in SageMathCloud: