What's wrong with:

if not isinstance(data, basestring):
    raise TypeError(...)

?

In any case, you appear to be wanting to use assert to check user input.  That's not its intended use;  instead, it's there for making debugging assertions.  Bear in mind that when running in optimized mode (with python -O), Python won't execute those asserts at all.  (See http://docs.python.org/3.4/reference/simple_stmts.html#the-assert-statement for more.)

I think this should be rejected.