Python : Having trouble with using the interpolate function

[mastermind12]

    # Imported array of data from a text file. This code works with no problems.
    q1, a1 = loadtxt("values.txt", unpack = True, skiprows = 1)
    print q1
    print a1

    # Creating a while loop for this part of the code. This code works with no problems.
    a = 3
    b = -2
    c = -9
    q = 0.5
    qt = 0.1

    while q < 1.5:
        print q, a
        q += qt
        a = a + b*qt
        b = b + c*qt

    # This interpolation does not work. I am unable to figure it out.
    # TypeError: object of type 'float' has no len()

    from scipy.interpolate import interp1d
    f = interp1d(q,a,'cubic')
    q1 = linspace(0.5,1.4,25)
    a1 = f(q1)
    plot(q1,a1, '-',  q,a, 'o')
    show()

Thanks.