Macro in mandatory argument of tblr of tabularray does not work

Question

Minimal working example:

%! TeX program = lualatex
\documentclass{article}
\usepackage{pyluatex}
\usepackage{tabularray}
\usepackage{xcolor}
\begin{document}
\let\rows\relax
\begin{python}
print("\global\def\rows{Alpha & Beta & Gamma & Delta \\  Epsilon & Zeta  & Eta & Theta \\ Iota & Kappa & Lambda  & Mu \\  Nu & Xi & Omicron & Pi \\}")
\end{python}
\let\trivgroups\relax
\begin{python}
mystr = ""
for i in range(1,5):
  mystr += 'cell{'+str(i)+'}{'+str(i)+'} = {red4},'
print("\global\def\trivgroups{"+mystr+"}")
\end{python}
\ExplSyntaxOff
% This works
% \begin{tblr}[expand=\rows]{colspec={|c|c|c|c|},
%     cell{1}{1}={red4},
%     cell{2}{2}={red4},
%     cell{3}{3}={red4},
%     cell{4}{4}={red4}
% }
\begin{tblr}[expand=\rows]{colspec={|c|c|c|c|},\trivgroups}
\rows
\end{tblr}
\end{document}

Compilation still works

> lualatex --output-format=pdf --shell-escape pythontex-tabularray.tex

However, no colour in the output:

The commented version gives

Follow-up question of Generate rows for tabularray with pyluatex with possible related questions:

• tabularray: Expand multiple macros
• tabularray: Reading a table stored in a macro by catchfile
• Misplaced alignment tab character &. with tabularray and expl3
• What is the best way to expand macros inside keyval input in expl3

UPDATE

Before I used the new environment exptblr from the answer by egreg, I could rotate the text in the header row as following:

\documentclass{article}
\usepackage{pyluatex}
\usepackage{tabularray}
\usepackage{xcolor}
\usepackage{rotating} 
\usepackage{makecell} 
\UseTblrLibrary{diagbox}
\setlength\rotheadsize{5cm}
\renewcommand\theadfont{}
\ExplSyntaxOn
\NewDocumentEnvironment{exptblr}{O{}m}
 {
  \use:x
   {
    \exp_not:N \begin{tblr}
    [\exp_not:n{#1}]
    {#2}
   }
 }
 {
  \end{tblr}
 }
\ExplSyntaxOff
\begin{document}
\let\rows\relax
\begin{python}
print("\global\def\rows{Alpha & Beta & Gamma & Delta \\ Epsilon & Zeta  & Eta & Theta \\ Iota & Kappa & Lambda  & Mu \\ Nu & Xi & Omicron & Pi \\}")
\end{python}
\begin{python}
mystr = ""
for i in range(2,5):
  mystr += 'cell{'+str(i)+'}{'+str(i)+'} = {red4},'
\end{python}
\begin{tblr}[expand=\rows]{
  colspec={|c|c|c|c|},
  cell{1}{2-Z} = {halign=c,cmd=\rothead},
  hlines,vlines,
  % \py{mystr},
}
\diagbox[height=4cm,width=4cm]{subgroups $H$}{supergroups $G$} &
G1 Very Long & G2 Very Long & G3 Very Long \
\rows
\end{tblr}
\end{document}

But when I now use the new environment exptblr, I get following error:

! Use of \@gape doesn't match its definition.
\@ifnextchar ...eserved@d =#1\def \reserved@a {#2}
                                                  \def \reserved@b {#3}\futu...
l.45 }
?

Answer 1

tblr doesn't expand macros in its mandatory argument.

You can define a new environment:

\documentclass{article}
\usepackage{pyluatex}
\usepackage{tabularray}
\usepackage{xcolor}
\ExplSyntaxOn
\NewDocumentEnvironment{exptblr}{O{}m}
 {
  \use:x
   {
    \exp_not:N \begin{tblr}
    [\exp_not:n{#1}]
    {#2}
   }
 }
 {
  \end{tblr}
 }
\ExplSyntaxOff
\begin{document}
\let\rows\relax
\begin{python}
print("\global\def\rows{Alpha & Beta & Gamma & Delta \\ Epsilon & Zeta  & Eta & Theta \\ Iota & Kappa & Lambda  & Mu \\ Nu & Xi & Omicron & Pi \\}")
\end{python}
\let\trivgroups\relax
\begin{python}
mystr = ""
for i in range(1,5):
  mystr += 'cell{'+str(i)+'}{'+str(i)+'} = {red4},'
print("\global\def\trivgroups{"+mystr+"}")
\end{python}
\begin{exptblr}[expand=\rows]{colspec={|c|c|c|c|},\trivgroups}
\rows
\end{exptblr}
\end{document}

As user187803 points out in comments this can be simplified with \py{mystr}. I added also a couple of changes: \let\rows\relax is not needed and \global\def can be \gdef.

\documentclass{article}
\usepackage{pyluatex}
\usepackage{tabularray}
\usepackage{xcolor}
\ExplSyntaxOn
\NewDocumentEnvironment{exptblr}{O{}m}
 {
  \use:x
   {
    \exp_not:N \begin{tblr}
    [\exp_not:n{#1}]
    {#2}
   }
 }
 {
  \end{tblr}
 }
\ExplSyntaxOff
\begin{document}
\begin{python}
print("\gdef\rows{Alpha & Beta & Gamma & Delta \\ Epsilon & Zeta  & Eta & Theta \\ Iota & Kappa & Lambda  & Mu \\ Nu & Xi & Omicron & Pi \\}")
\end{python}
\begin{python}
mystr = ""
for i in range(1,5):
  mystr += 'cell{'+str(i)+'}{'+str(i)+'} = {red4},'
\end{python}
\begin{exptblr}[expand=\rows]{colspec={|c|c|c|c|},\py{mystr}}
\rows
\end{exptblr}
\end{document}