def adder(a,b):  # function to test.
    """ adds a to b"""
    return a+b

options = [9, 9.0, [9], "9"]  # options for test function


test_template = """

def test_adder_{}():
    _ = adder{}
"""   # -- template for test that fail --


def test_discovery():
    from pathlib import Path
    from itertools import product

    test_number = 0
    for c in product(*[options, options]):
        test_number += 1
        try:
            output = adder(*c)
        except Exception:
            new_test = test_template.format(test_number, c)  # write up test.
            with Path(__file__).open('a') as fo:  # append it to the file.
                fo.write(new_test)


test_discovery()  # main callable used to populate tests.

# test that fail will be written from here.