CODEWITHSUNDEEP
pythonpython-2.7
MicArn
MicArn

Reputation: 9

I keep getting a " TypeError: unsupported operand type(s) for -: 'NoneType' and 'NoneType' "

I'm learning python and as practice I wrote some code to find the derivative of a user defined function. The code is as follows. 

def fx(value, function):
    x = value
    return eval(function)


input_function = raw_input('Input your function using correct python syntax: ')

def derivative_formula(x, h):
    (fx(x - h, input_function) - fx(x + h, input_function)) / 2 * h

def derivative_of_x(x):
    error = 0.0001
    h = 0.1
    V = derivative_formula(x, h)
    h = h / 2
    derivative_estimate = derivative_formula(x, h)
    while abs(derivative_estimate - V) < error:
        V = derivative_formula(x, h)
        h = h / 2
        derivative_estimate = derivative_formula(x, h)
    return derivative_estimate

x1 = float(raw_input('Where do you want to calculate the derivative?'))

return derivative_of_x(x1)

The full error code is as follows

Traceback (most recent call last):
    File "Derivative.py", line 25, in <module>
        print derivative_of_x(x1)
    File "Derivative.py", line 17, in derivative_of_x
        while abs(derivative_estimate - V) - error:
TypeError: Unsupported operand type(s) for -: 'NoneType' and 'NoneType'

Upvotes: 1

Views: 729

Answers (2)

Kevin
Kevin

Reputation: 76194

You forgot to return anything from your derivative_formula function. Try:

def derivative_formula(x, h):
    return (fx(x - h, input_function) - fx(x + h, input_function)) / 2 * h

Also, if you are trying to divide by 2h, you'll need an additional parenthesis.

def derivative_formula(x, h):
    return (fx(x - h, input_function) - fx(x + h, input_function)) / (2 * h)

And I think you want to reverse the two function calls there.

def derivative_formula(x, h):
    return (fx(x + h, input_function) - fx(x - h, input_function)) / (2 * h)

Upvotes: 2

JoErNanO
JoErNanO

Reputation: 2488

Your derivative_formula has no return statement which in python defaults to return None. Hence when you call abs(derivative_estimate - V) you are effectively subtracting a NoneType object (V) from another NoneType object.

What you need is the return statement in derivative_formula:

def derivative_formula(x, h):
    res = (fx(x - h, input_function) - fx(x + h, input_function)) / 2 * h
    return res

Upvotes: 0

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