Save and restore the makeatletter/makeatother state

This is what—in some situations—I do:

\begingroup
\makeatletter
\@firstofone{%
  \endgroup
  ⟨Stuff/macro-definitions whose tokenizing needs to
   take place while `@` is of category-code 11(letter)
   but whose processing/carrying-out/running/execution
   is to take place while the category-code of `@` is
   what it was before encountering \makeatletter.⟩
}%

The argument of \@firstofone will be read and tokenized within a local scope where @ is of category-code 11(letter) and where therefore @ either can be part of the name of control-word-tokens or will be tokenized as an explicit character-token of category-code 11(letter).
When processing/carrying out/running/executing those tokens that came into being due to reading and tokenizing the argument of \@firstofone takes place, the first \endgroup will terminate that local scope, resetting the category code of @ to whatever it was before calling \makeatletter.
Thus processing/carrying out/running/executing the remaining tokens of \@firstofone's argument does not take place within that local scope where the category-code of @ is changed but with the category-code-régime that was in effect before processing the \makeatletter-token.

You could define a new macro to store the current catcode of @ and restore it using this macro later on:

\documentclass{article}

\def\safemakeatletter#1{%
    \edef#1{\catcode`@=\the\catcode`@\relax}%
    \makeatletter
}

\def\testat{catcode of \texttt{@} is: \the\catcode`@\relax\par}

\begin{document}

\catcode`@=10\relax
\testat
\makeatletter
\testat
\makeatother
\testat

\medskip

\catcode`@=10\relax
\testat
\safemakeatletter\safemakeatother
\testat
\safemakeatother
\testat

\end{document}

As you can see in the output, \makeatother incorrectly restores the catcode in the first part, while \safemakeatother doesn't: