Python object of type 'float' has no len()

Question

this is the solution to a problem I'm working on (learning Python), but even the solution is giving me an error.

Here's the code:

def compute_deriv(poly):

    """
    Computes and returns the derivative of a polynomial function. If the
    derivative is 0, returns [0.0].

    Example:
    >>> poly = [-13.39, 0.0, 17.5, 3.0, 1.0]    # - 13.39 + 17.5x^2 + 3x^3 + x^4
    >>> print compute_deriv(poly)        # 35^x + 9x^2 + 4x^3
    [0.0, 35.0, 9.0, 4.0]

    poly: list of numbers, length > 0
    returns: list of numbers
    """

    poly_deriv = []
    if len(poly) < 2:
        return [0.0]
    for j in xrange(1, len(poly)):
        poly_deriv.append(float(j * poly[j]))
    return poly_deriv

This is the solution given to me, but when I use the following code to call the function:

poly1 = (-13.39)
print compute_deriv(poly1)

I get

TypeError: object of type 'float' has no len()

I've tried a couple different things inside the if statement (since this code only breaks when len(poly) is < 2;

I tried poly_deriv.append(0.0) and return poly_deriv, for example.

Answer 1

I'm guessing there's a typo in the solution, and they intended it to be a one element collection.

poly1 = (-13.39,)

Or

poly1 = [-13.39]

The way they have it right now, it's a single float rather than a collection containing one float. (-13.39) is syntactically equivalent to just -13.39 with no parentheses.

Answer 2

The line poly1 = (-13.39) isn't a tuple. Python reads this as a parenthetical operation not a so your variable is simply a number. Try a more explicit definition instead:

poly1 = tuple(-13.39)