When writing a class/package, how can I run code if/when another package is loaded? [duplicate]

Question

I'm writing a thesis class file and I'd like to delay the running of code to after a specific package is loaded:

\AtEndOfOtherPackage{listings}{%
  \lstset{language=Python}}

Is there a command that already exists for the task? If there isn't, how could it be effected?

Answer 1

With scrlfile you get \AfterPackage, that's executed only if the package is loaded, and at the end of it. So

\RequirePackage{scrlfile}

\AfterPackage{listings}{\lstset{language=Python}}

should be what you want.

Answer 2

If you need the package to be loaded, do as Herbert showed,

\RequirePackage{listings}
\lstset{language=Python}

If you want to conditionally run stuff after the package if the user loads that in the preamble then there is not (by default) an end of package hook usable for this (The hook used by \AtEndOfPackage is cleared as the package is loaded) So the normal thing is to use the begin document hook

\AtBeginDocument{\@ifpackageloaded{listings}%
   {\typeout{yes}}%
   {\typeout{no}}%
 }

Answer 3

\RequirePackage{listings}
\lstset{language=Python}

If it is already loaded before \RequirePackage nothing happens and \lstset will be used as usual.