Question

how do i assert that a variable in a function (say f(x)) is a string?

Answer 1

Within the function definition, write the following line (where inputstring is the variable to be tested):

assert type(inputstring) is str, 'inputstring is not a string.'

If inputstring was a string, the function proceeds.
If inputstring was anything else, the interpreter throws an AssertionError at you and prints the string that follows the comma in the assertion statement.

Answer 2

if type(thing) is not str:
raise TypeError, thing + ' is not a string'