[LaTeX] How to tell the user that the document wasn't compiled with -shell-escape flag?

Hi,

I am writing a LaTeX package and it invokes external Python code to do
some heavy preprocessing. I have not learned TeX programming language
enough yet to do everything in TeX. So, when the user of my package
doesn't invoke pdflatex with the -shell-escape flag, I would like print
a helpful error message telling them so.

Anybody has any idea how that can be done. Thank you.

    Annada