"""
Exercise 3.6 from "A primer on..."
The test function is given, write the function
so that it passes.
"""

def double(a):
    return a * 2


def test_double():
    assert double(2) == 4
    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()