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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.