how can I check if a variable is a structure (i.e. a list)? For my special problem the variable is either a character string OR a list of character strings line ['word1', 'word2',...]
You're trying to apply the LBYL principle. My bet is that your "special problem" can be solved by the EAFP principle. (These terms are explained in the Glossary of the tutorial, <http://www.python.org/doc/current/tut/node18.html>.)
If you test for a specific set of types, your code will not work with
types that you have not considered yet behave like lists. As you discover more object types that you want the code to work with, it will sprout more cruft for checking those types.
If you want to use some list behaviour of the object, don't check first. Use it, and catch the TypeError exception in the event that it's not. This way, *any* object that implements the functionality you need will work, regardless of its type. -- http://mail.python.org/mailman/listinfo/python-list
