Can't pass argument to second part of newenvironment [duplicate]

Question

This code works:

\documentclass{book}

\newenvironment{testenvironment}[1][]
    {\begin{quote}#1}
    {\end{quote}}

\begin{document}

\begin{testenvironment}[testargument]
test text
\end{testenvironment}

\end{document}

This code doesn't:

\documentclass{book}

\newenvironment{testenvironment}[1][]
    {\begin{quote}}
    {#1\end{quote}}

\begin{document}

\begin{testenvironment}[testargument]
test text
\end{testenvironment}

\end{document}

The only difference is whether #1 is written within the first vs. the second bracket of the \newenvironment command.

How do I pass the argument to the second bracket, to the content after the input?

Answer 1

With more recent LaTeX-releases you can use \NewDocumentEnvironment instead of \newenvironment.
You may need to load the package xparse.

\documentclass{book}
\usepackage{graphicx}

\NewDocumentEnvironment{testenvironment}{O{}}
    {\begin{quote}}
    {#1\end{quote}}

\begin{document}

\begin{testenvironment}[testargument]
test text
\end{testenvironment}

\begin{testenvironment}
test text
\end{testenvironment}

\begin{testenvironment}[testargumentB]
test text
\end{testenvironment}

\end{document}