Patching a command with an optional argument or declared with \DeclareRobustCommand

Question

To re-define a command to contain itself (patching in TeX parlance) you usually use \let. This answer by @egreg says:

Never use \let on commands defined with \DeclareRobustCommand or commands defined with \newcommand and having an optional argument.

So a natural question is what should we do in such cases?

For example, how do I re-define \bibitem to add another command after it? This:

\let\old@bibitem\bibitem
\def\bibitem{\old@bibitem\it}

does not work, and \show\bibitem shows why:

> \bibitem=macro:
->\@ifnextchar [\@lbibitem \@bibitem .

What should I do to achieve the effect?

I would prefer not to use packages such as xpatch because I am modifying a class which others will use, and I try to use as few pacages as possible to minimize possible conflicts.

Answer 1

The command \bibitem has no argument, much less an optional one. The commands I refer to in the quoted part of my answer are those defined with

\newcommand{<macro>}[<args>][<default>]{...}

not others defined in an indirect way with \@ifnextchar or \@ifstar like \bibitem or \section.

If your aim is to print the citation label in italics, patching \bibitem is the wrong choice independently of any package such as etoolbox, xpatch or letltxmacro you load.

What could you do? Let's see: \bibitem has the definition you showed, so we know we have to patch either \@lbibitem or \@bibitem.

Using texdef we see

> texdef -t latex -s @lbibitem
% latex.ltx, line 6228:
\def\@lbibitem[#1]#2{\item[\@biblabel{#1}\hfill]\if@filesw
      {\let\protect\noexpand
       \immediate
       \write\@auxout{\string\bibcite{#2}{#1}}}\fi\ignorespaces}

> texdef -t latex -s @bibitem
% latex.ltx, line 6232:
\def\@bibitem#1{\item\if@filesw \immediate\write\@auxout
       {\string\bibcite{#1}{\the\value{\@listctr}}}\fi\ignorespaces}

Well. There are two cases: if \bibitem is followed by [, we need to see how \@biblabel is defined and we see

> texdef -t latex -s @biblabel
% latex.ltx, line 6278:
\def\@biblabel#1{[#1]}

so we just need to do

\def\@biblabel#1{[\textit{#1}]}

If there is no optional argument, we need to see how the standard label of the enumerate environment started with \begin{thebibliography} is defined. Very well:

> texdef -t latex -s thebibliography
% article.cls, line 575:
\newenvironment{thebibliography}[1]
     {\section*{\refname}%
      \@mkboth{\MakeUppercase\refname}{\MakeUppercase\refname}%
      \list{\@biblabel{\@arabic\c@enumiv}}%
           {\settowidth\labelwidth{\@biblabel{#1}}%
            \leftmargin\labelwidth
            \advance\leftmargin\labelsep
            \@openbib@code
            \usecounter{enumiv}%
            \let\p@enumiv\@empty
            \renewcommand\theenumiv{\@arabic\c@enumiv}}%
      \sloppy
      \clubpenalty4000
      \@clubpenalty \clubpenalty
      \widowpenalty4000%
      \sfcode`\.\@m}
     {\def\@noitemerr
       {\@latex@warning{Empty `thebibliography' environment}}%
      \endlist}

Hmm, this depends on the class; let's assume the class is article. Fine! The label is still printed using \@biblabel, so the above trick is good in both cases.

So your task is just adding

\def\@biblabel#1{[\textit{#1}]}

to your personal style file; if the patch must be done in the document preamble, ensure to add the line between \makeatletter and \makeatother (see What do \makeatletter and \makeatother do?). However this might depend on the class, but usually one can count on the fact that \@biblabel is what's to be changed.

No, patching commands is definitely not easy.