Writing enumitem's to a file

Question

I am a teacher and often need to make worksheets and reviews that have answer keys. With the help of TeX StackExchange, I have a fairly robust system at this point with the notable exception that it does not work with enumitem. The error message that I am getting is beyond my LaTeX wisdom. Is there anyone who can help me out?

MWE:

\documentclass[10pt]{article}

\usepackage{pgfplots}
\usepackage{enumitem}

\newwrite\answerkeyfile

\AtBeginDocument{%
   \immediate\openout\answerkeyfile=\jobname.keys
}

\begingroup
\catcode`\#=12
\gdef\hashchar{#}%
\endgroup

\makeatletter
\newcommand{\answerkey}[1]{%
  \begingroup
  \let\#\hashchar
  \immediate\write\answerkeyfile{\@currentlabel.\ }
  \immediate\write\answerkeyfile{\unexpanded{#1}}%
  \immediate\write\answerkeyfile{ }
  \endgroup
}
\makeatother

\AtEndDocument{%
  \immediate\closeout\answerkeyfile
}

\begin{document}

Answer these questions:

%% With \begin{enumerate}, this code works.  Switch to 
%% \begin{enumerate}[label=\emph{\alph*)}] and it fails.
%\begin{enumerate}
\begin{enumerate}[label=\emph{\alph*)}]
   \item Question 1 \answerkey{Answer 1}
   \item Question 2 \answerkey{$\frac{1}{2}$}
   \item Question 3 \answerkey{\begin{tikzpicture}
                          \draw (0,0) rectangle (1,1);
                          \end{tikzpicture}}
\end{enumerate}

Answer Key:

% Copy the mwe.keys to mwe.ans, then use this to show the answers.
%\input{mwe.ans}

\end{document}

As noted in the comments, switch the enumerate line to the one that uses enumitem features and this will fail. What can I do to not compromise the system I have and yet still use enumitem?

Here is the error that I am getting:

! Use of \@item doesn't match its definition. \text@command #1->\def \reserved@a {#1}\ifx \reserved@a \@empty \let \check@... l.37 \item Question 1 \answerkey{Answer 1}

UPDATE:

The comment below that it is not actually the enumitem code but rather the \emph (or similarly \textbf, \textit, etc) in the label that is the issue was interesting. The solution below that creates a "fake" label is the least invasive/complicated, while adding the functionality that label formatting makes it through to the answer key document. The code I started with did not put the answers into a true enumerate environment, so the solution proposed below which does this is overkill for my situation. In the end, based on the help/guidance below, I rewrote the \answerkey command as follows:

\newcommand{\answerkey}[2][.]{%
  \begingroup
  \let\#\hashchar
  \immediate\write\answerkeyfile{\expandafter\unexpanded
         \expandafter{\@currentlabel}#1\ }
  \immediate\write\answerkeyfile{\unexpanded{#2}}%
  \immediate\write\answerkeyfile{ }
  \endgroup
}

In actual use, it looks like this:

If the default item labels are used:

\begin{enumerate}
    \item Question 1 \answerkey{Answer 1}

If enumitem label formatting is used:

\begin{enumerate}[label={\arabic*.}]
    \item Question 1 \answerkey[]{Answer 1}

Answer 1

You should use the ref feature of enumitem, also exploiting the fact that the package braces the \@currentlabel.

Define \answeritem as you like.

\documentclass[10pt]{article}

\usepackage{pgfplots}
\usepackage{enumitem}

\newwrite\answerkeyfile

\AtBeginDocument{%
   \immediate\openout\answerkeyfile=\jobname.keys
}

\edef\hashchar{\string#}

\makeatletter
\newcommand{\answerkey}[1]{%
  \begingroup
  \let\#\hashchar
  \immediate\write\answerkeyfile{\answeritem\@currentlabel}
  \immediate\write\answerkeyfile{\unexpanded{#1}^^J}%
  \endgroup
}
\protected\def\answeritem#1{\item[\emph{#1})]}
\makeatother

\begin{document}

Answer these questions:

%% With \begin{enumerate}, this code works.  Switch to 
%% \begin{enumerate}[label=\emph{\alph*)}] and it fails.
%\begin{enumerate}
\begin{enumerate}[label=\emph{\alph*}),ref=\alph*]
   \item Question 1 \answerkey{Answer 1}
   \item Question 2 \answerkey{$\frac{1}{2}$}
   \item Question 3 \answerkey{\begin{tikzpicture}
                          \draw (0,0) rectangle (1,1);
                          \end{tikzpicture}}
\end{enumerate}

Answer Key:

\immediate\closeout\answerkeyfile
\begin{enumerate}
\input{\jobname.keys}
\end{enumerate}

\end{document}

Here's the output in \jobname.keys:

\answeritem {a}
Answer 1

\answeritem {b}
$\frac {1}{2}$

\answeritem {c}
\begin {tikzpicture} \draw (0,0) rectangle (1,1); \end {tikzpicture}

Some more words about the problem.

The key ref defines what's stored in \@currentlabel (typically written in the .aux file, but you're using it yourself). Writing in the .aux file is safe, because \protected@write is used by LaTeX; however you're using \write and \emph is not a command TeX likes to see there. You could use \protected@iwrite (you find how to define it on the site), but the trick I suggest above is even better because you can then decide what \answeritem gets expanded to. It's important its definition is \protected, so it will remain untouched during the \write operation.

Answer 2

I suggest to expand \@currentlabel and write that content to the file with \unexpanded, however, this will place 'fake' item labels and not really \item layout in the file.

A better strategy would be to 'replicate' the enumerate environment in the output file.

\documentclass[10pt]{article}

\usepackage{pgfplots}
\usepackage{enumitem}

\newwrite\answerkeyfile

\AtBeginDocument{%
   \immediate\openout\answerkeyfile=\jobname.keys
}

\begingroup
\catcode`\#=12
\gdef\hashchar{#}%
\endgroup

\makeatletter
\newcommand{\answerkey}[1]{%
  \begingroup
  \let\#\hashchar
  \immediate\write\answerkeyfile{\expandafter\unexpanded\expandafter{\@currentlabel}.\ }
  \immediate\write\answerkeyfile{\unexpanded{#1}}%
  \immediate\write\answerkeyfile{ }
  \endgroup
}
\makeatother

\AtEndDocument{%
  \immediate\closeout\answerkeyfile
}

\begin{document}

Answer these questions:

%% With \begin{enumerate}, this code works.  Switch to 
%% \begin{enumerate}[label=\emph{\alph*)}] and it fails.
%\begin{enumerate}
\begin{enumerate}[label=\emph{\alph*)}]
   \item Question 1 \answerkey{Answer 1}
   \item Question 2 \answerkey{$\frac{1}{2}$}
   \item Question 3 \answerkey{\begin{tikzpicture}
                          \draw (0,0) rectangle (1,1);
                          \end{tikzpicture}}
\end{enumerate}

Answer Key:

% Copy the mwe.keys to mwe.ans, then use this to show the answers.
\immediate\closeout\answerkeyfile
\input{\jobname.keys}

\end{document}

Another approach by writing \begin{answerenumerate} to the file, which is a copy of enumerate, basically:

\documentclass[10pt]{article}

\usepackage{pgfplots}
\usepackage{enumitem}

\newwrite\answerkeyfile

\AtBeginDocument{%
   \immediate\openout\answerkeyfile=\jobname.keys
}

\begingroup
\catcode`\#=12
\gdef\hashchar{#}%
\endgroup

\makeatletter
\newcommand{\answerkey}[1]{%
  \begingroup
  \let\#\hashchar
  \immediate\write\answerkeyfile{\string\item}
  \immediate\write\answerkeyfile{\unexpanded{#1}}%
  \immediate\write\answerkeyfile{ }
  \endgroup
}
\makeatother

\AtEndDocument{%
  \immediate\closeout\answerkeyfile
}

\begin{document}

Answer these questions:

\newlist{answerenumerate}{enumerate}{1}
\setlist[answerenumerate]{label=\emph{\alph*)}}


\setlist[enumerate,1]{label=\emph{\alph*)},
  before={\immediate\write\answerkeyfile{\string\begin{answerenumerate}}},
    after={\immediate\write\answerkeyfile{\string\end{answerenumerate}}}
}

\begin{enumerate}
   \item Question 1 \answerkey{Answer 1}
   \item Question 2 \answerkey{$\frac{1}{2}$}
   \item Question 3 \answerkey{\begin{tikzpicture}
                          \draw (0,0) rectangle (1,1);
                          \end{tikzpicture}}
\end{enumerate}

Answer Key:

% Copy the mwe.keys to mwe.ans, then use this to show the answers.
\immediate\closeout\answerkeyfile
\input{\jobname.keys}

\end{document}