\itemize vs. \begin{itemize} not behaving as expected

Question

I want to write a wrapper macro for an itemize environment. I would like to use the enumitem package (though the MWE here doesn't use or need this package). The problem is the parameters that I'm passing to the itemize environment may themselves be encoded inside another macro.

So for the wrapper macro, I wanted to write something like

\expandafter\itemize\expandafter[\anothermacro]

to handle this. But I got very unexpected results.

For some reason, the following two styles to embedding an itemize environment in a wrapper macro don't work identically.

The first style just uses \begin{itemize} followed by content and then a closing \end{itemize}.

The second style used \itemize followed by content and then a closing \enditemize. But in the second style, I get nested environments. Not at what I'm expecting.

\documentclass{article}
\pagestyle{empty}
\usepackage{xparse}
\ExplSyntaxOn
\clist_new:N \l_ae_itemize_clist
%%--------------------------------------------------------------------------------
%% Style A                                                                        
\cs_new:Npn \_ae_itemize_a:nn #1#2 
  {
    \clist_set:Nn \l_ae_itemize_clist { #2 }
     \begin{itemize}
    \clist_map_inline:Nn \l_ae_itemize_clist
      { \item ##1 }
    \end{itemize}
  }

\NewDocumentCommand{\aefooda}{ O{}mm }
    { \par\textbf{#2:}
      \_ae_itemize_a:nn {#1}{#3}
    }
%%--------------------------------------------------------------------------------
%% Style B                                                                        
\cs_new:Npn \_ae_itemize_b:nn #1#2 
  {
    \clist_set:Nn \l_ae_itemize_clist { #2 }
     \itemize
    \clist_map_inline:Nn \l_ae_itemize_clist
      { \item ##1 }
    \enditemize
  }

\NewDocumentCommand{\aefoodb}{ O{}mm }
    { \par\textbf{#2:}
      \_ae_itemize_b:nn {#1}{#3}
    }

\ExplSyntaxOff
\begin{document}

\textbf{Style A} works fine:
\aefooda{first}{a,b,c}

\aefooda{second}{a,b,c}

\aefooda{third}{a,b,c}

%%--------------------------------------------------------------------------------
%% Changing from style A to style B                                               
%%--------------------------------------------------------------------------------

\textbf{Style B} doesn't work:
\aefoodb{first}{a,b,c}

\aefoodb{second}{a,b,c}

\aefoodb{third}{a,b,c}

\end{document}

UPDATE

I've also tried something like the following:

\edef\aeBeginItemize[#1]{\noexpand\begin{itemize}[#1]}
\cs_new:Npn \_ae_itemize_c:nn #1#2 
  {
    \clist_set:Nn \l_ae_itemize_clist { #2 }
    \aeBeginItemize[#1]
    \clist_map_inline:Nn \l_ae_itemize_clist
      { \item ##1 }
    \end{itemize}
  }

But when I try to use it, I get an error from the enumitem package: it seems that enumitem is not longer able to process the keys correctly.

Answer 1

itemize needs to know the nesting level it is, so it increments (locally) a counter. Within \begin{itemize} and \end{itemize} there are \begingroup and \endgroup that keep that assignment local. You surely can emulate it by adding \group_begin: and \group_end: to your function; choose between style A or B, where I show also how you can expand the option list for itemize:

\documentclass{article}
\usepackage{enumitem}
\usepackage{xparse}
\pagestyle{empty}

\ExplSyntaxOn
\clist_new:N \l_ae_itemize_clist

%%--------------------------------------------------------------------------
%% Style A                                                                        
\cs_new:Npn \_ae_itemize_a:nn #1#2 
  {
   \clist_set:Nn \l_ae_itemize_clist { #2 }
   \use:x { \exp_not:n { \begin{itemize} } [\exp_not:o {#1}] }
   \clist_map_inline:Nn \l_ae_itemize_clist { \item ##1 }
   \end{itemize}
  }

\NewDocumentCommand{\aefooda}{ O{}mm }
    { \par\textbf{#2:}
      \_ae_itemize_a:nn {#1}{#3}
    }
%%--------------------------------------------------------------------------
%% Style B                                                                        
\cs_new:Npn \_ae_itemize_b:nn #1#2 
  {
   \group_begin:
   \clist_set:Nn \l_ae_itemize_clist { #2 }
   \use:x { \exp_not:N \itemize [ \exp_not:o { #1 } ] }
   \clist_map_inline:Nn \l_ae_itemize_clist { \item ##1 }
   \enditemize
   \group_end:
  }

\NewDocumentCommand{\aefoodb}{ O{}mm }
    { \par\textbf{#2:}
      \_ae_itemize_b:nn {#1}{#3}
    }

\ExplSyntaxOff
\begin{document}

\textbf{Style A} works fine:
\aefooda{first}{a,b,c}

\aefooda{second}{a,b,c}

\aefooda{third}{a,b,c}

%%--------------------------------------------------------------------------
%% Changing from style A to style B                                               
%%--------------------------------------------------------------------------

\textbf{Style B} works too:
\aefoodb{first}{a,b,c}

\aefoodb[label=+]{second}{a,b,c}

\newcommand{\test}{label=X,font=\textbf}
\aefoodb[\test]{third}{a,b,c}

\end{document}