Python TypeError float argument must be string or number not list

Here is my block of code:

x = [1.6, 2, 2.5, 3.2, 4, 4.5]
y = [2, 8, 14, 15, 8, 2]
r = 2.8
e = []


def err(x, y, r):
    n = len(x) - 1
    a = y.copy()
    for i in range(1, n + 1):
        for j in range(n, i - 1, -1):
            a[j] = float(a[j] - a[j-1])/float(x[j] - x[j-i])
    for k in range(n):
        e[k] = float(a[0:k])*float(x[0:k] - r)
    return e

It's the error calculation for Newton polynomial interpolation. Here's the complete error message:

TypeError                                 Traceback (most recent call last)
<ipython-input-3-dafb0d988dac> in <module>()
----> 1 err(x, y, r)

<ipython-input-2-7d51080e7243> in err(x, y, r)
      9             a[j] = float(a[j] - a[j-1])/float(x[j] - x[j-i])
     10     for k in range(n):
---> 11         e[k] = float(a[0:k])*float(x[0:k] - r)
     12     return e

TypeError: float() argument must be a string or a number, not 'list'

I've been at this for hours, have not been able to resolve this issue. I hope someone here can help (I'm relatively new to Python). Thanks in advance.

The error calculation for Newton interpolation is:

a0*(r-x0), 
a1*(r-x0)*(r-x1), 
a1*(r-x0)*(r-x1)*(r-x2) 
.... 
a5*(r-x0)*(r-x1)*(r-x2)*(r-x3)*(r-x4)*(r-x5)

(for this case, it runs to a5 for a 5th degree polynomial).
The "a" values are contained in list a. Also, x is a fixed value (r in this case = 2.8), and x0, x1, x2, etc. are from x = [1.6, 2, 2.5, 3.2, 4, 4.5]