Limit macro/newcommand scope to section

Question

How can I limit the scope of a \newcommand to a \section? This is what I'd like to have:

1. The command is defined only inside the \section, not globally;
2. The command scope is that \section and cannot be seen outside of it;
3. It should not matter if a \subsection sees the command or not (it probably makes sense that it sees it as well); and
4. The command can be defined again in a different \section without generating errors (I know that \renewcommand could solve it but I'd like to use \newcommand if possible).

What I have is (which generates an error because of the two definitions):

\documentclass{article}
\begin{document}

\section{First section}
\newcommand*{\blipo}{Jill}
Hello \blipo{}. \blipo{} is blonde.

\section{Second section}
\newcommand*{\blipo}{ET}
Hello \blipo{}. \blipo{} is green.

\section{Third section}
Not defined here. Attempt to use it generates error.

\end{document}

[edit] In my particular case, I'll be having around 20-30 different \newcommand in a single section which may or may not be used (redefined) in different sections of the document.

Answer 1

It seems that this can simply be solved by enclosing the scope with braces. A minimal example could look like:

\documentclass[10pt,a4paper]{article}
\begin{document}

\section{Section A}
{
\newcommand{\sectionowner}{me}
This section belongs to \sectionowner
}
\section{Section B}
{
\newcommand{\sectionowner}{you}
This section belongs to \sectionowner
}

\end{document}

where the command \sectionowner is defined two times using \newcommand.

Answer 2

You can add code to \section so that \blipo gets undefined:

\documentclass{article}
\usepackage{etoolbox}
\preto\section{\undef\blipo}
\begin{document}
\section{First section}
\newcommand*{\blipo}{Jill}
Hello \blipo{}. \blipo{} is blonde.
\section{Second section}
\newcommand*{\blipo}{ET}
Hello \blipo{}. \blipo{} is green.
\section{Third section}
Not defined here. Attempt to use it generates error.
\blipo{}
\end{document}

Here's a piece of the terminal session:

! Undefined control sequence.
l.19 \blipo
           {}
?

---

For a more general version, where you want several commands to get undefined at each section, use the list processing functions of etoolbox:

\documentclass{article}
\usepackage{etoolbox}
%%% The helper macros
\newcommand{\onlysectioncommands}[1]{%
  \forcsvlist{\listadd\sectioncommandslist}{#1}}
\preto\section{%
  \forlistloop{\undef}{\sectioncommandslist}}
%%% Here we set the special commands; the
%%% list can be augmented anywhere
\onlysectioncommands{\blipo,\foo}
\begin{document}
\section{First section}
\newcommand*{\blipo}{Jill}
\newcommand{\foo}{foo}
Hello \blipo{}. \blipo{} is blonde. \foo
\section{Second section}
\newcommand*{\blipo}{ET}
\newcommand{\foo}{bar}
Hello \blipo{}. \blipo{} is green. \foo
\section{Third section}
\newcommand{\foo}{baz}
Not defined here. Attempt to use it generates error.
\blipo{}
\foo
\end{document}

---

With \preto we add code at the start of \section. If the class or packages you're using redefine \section, you may need to use

\usepackage{xpatch}
\xpretocmd{\section}
  {\forlistloop{\undef}{\sectioncommandslist}}
  {}{}

instead.

Update 2023

With the current features of the LaTeX kernel:

\documentclass{article}
\ExplSyntaxOn
\clist_new:N \g_daniel_onlysection_commands_clist
\NewDocumentCommand{\onlysectioncommands}{m}
 {
  \clist_gput_right:Nn \g_daniel_onlysection_commands_clist { #1 }
 }
\AddToHook{cmd/section/before}
 {
  \clist_map_function:NN \g_daniel_onlysection_commands_clist \cs_undefine:N
 }
\ExplSyntaxOff
%%% Here we set the special commands and
%%% the list can be augmented anywhere
\onlysectioncommands{\blipo,\foo}
\begin{document}
\section{First section}
\newcommand*{\blipo}{Jill}
\newcommand{\foo}{foo}
Hello \blipo{}. \blipo{} is blonde. \foo
\section{Second section}
\newcommand*{\blipo}{ET}
\newcommand{\foo}{bar}
Hello \blipo{}. \blipo{} is green. \foo
\section{Third section}
\newcommand{\foo}{baz}
\verb|\blipo| is not defined here, attempt to use it generates error.
\blipo{}
\foo
\end{document}

Answer 3

Use \renewcommand:

\documentclass{article}
\begin{document}

\section{First section}
\newcommand*{\blipo}{Jill}
Hello \blipo{}. \blipo{} is blonde.

\section{Second section}
\renewcommand*{\blipo}{ET}
Hello \blipo{}. \blipo{} is green.

\section{Third section}
Not defined here. Attempt to use it generates error.

\end{document}