"""
Exercise 3.6 from "A primer on..."
Implement a function so that a given test function passes.
In this case the function itself is trivial, but the
exercise is an example of test-driven development, 
where the test is written first and then the function
is written to satisfy the test. 
"""

def double(arg):
    return arg * 2

def test_double():
    assert double(2) == 4
    assert double(0.1) == 0.2
    assert abs(double(0.1) - 0.2) < 1E-15
    assert double([1, 2]) == [1, 2, 1, 2]
    assert double((1, 2)) == (1, 2, 1, 2)
    assert double(3+4j) == 6+8j
    assert double('hello') == 'hellohello'

test_double()