Step 1: https://www.google.com/search?q=list+python+string+methods

Step 2: Parse the output with your eyes. The method is literally called "find".

This one-trick pony failure mode could perhaps have been fine for a guy who did Java and nothing but Java for 10 years, but you are supposedly the person who runs "pythonforengineers" website...

100% correct call by the interviewer.