Argument of setkeys

Question

My motivation bases on the package listings (also my package mdframed). The package allows to define a style via \lstset{foo}{options}. Now I want to create a macro to expand the style foo. For example \Apptolstset{foo}{more options}.

I created the followings example.

\documentclass{article}
\usepackage{filecontents}
\begin{filecontents}{testpackage.sty}
\ProvidesPackage{testpackage}
\RequirePackage{etoolbox,kvoptions}
\SetupKeyvalOptions{family=test,prefix=test@}
\DeclareStringOption[\bfseries]{font}
\DeclareStringOption[\normalsize]{size}
\ProcessKeyvalOptions*\relax
\newcommand\testsetup{\setkeys{test}}

\define@key{test}{loadstyle}{%
  \ifcsundef{test@style@#1}{}%
        {\expandafter\testsetup\expandafter{\csname test@style@#1\endcsname}}%
}
\newrobustcmd*\teststyle[2]{%
  \csdef{test@style@#1}{#2}%
}

\newrobustcmd*\apptodefinestyle[2]{%
 \ifcsundef{test@style@#1}%
   {}%
   {\csappto{test@style@#1}{,#2}}%
}


\newrobustcmd*\testresult[1]{{\test@font\test@size #1}}
\end{filecontents}
\usepackage{testpackage}
\begin{document}
Hello World!
\testresult{Hello World!}

\teststyle{new}{font=\bfseries\itshape}
\apptodefinestyle{new}{size=\Large}
\testsetup{loadstyle=new}

\testresult{Hello World!}

\end{document}

The error of this example is:

! Missing \endcsname inserted.
<to be read again>
\protect
l.35 \testsetup{loadstyle=new}
The control sequence marked <to be read again> should
not appear between \csname and \endcsname.

If I use the combination

\define@key{test}{loadstyle}{%
  \ifcsundef{test@style@#1}{}%
        {\csuse{test@style@#1}}%
}
\newrobustcmd*\teststyle[2]{%
  \csdef{test@style@#1}{\testsetup{#2}}%
}

everything will work. But with this combination I don't know how to define the command \apptoteststyle which should be expand the current style.

I hope the question is clear.

Answer 1

The \setkeys macro doesn't expand its second argument. If you have a macro which holds the keys you need to expand it first. Note that \csuse adds one expansion step (as any macro does) so you should use the underlying \csname ... \endcsname directly. This must be expanded twice, once to create the macro from its name and then to expand the macro itself.

So do:

\expandafter\expandafter\expandafter
\testsetup\expandafter\expandafter
\expandafter{\csname test@style@#1\endcsname}