python - 我不断收到 "TypeError: unsupported operand type(s) for -: ' NoneType' 和 'NoneType' ”

Question

我正在学习Python，作为练习，我编写了一些代码来查找用户定义函数的导数。代码如下。

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)

完整错误代码如下

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'

Answer 1

您忘记从 derivative_formula 函数返回任何内容。尝试:

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

此外，如果您尝试除以 2h，则需要一个额外的括号。

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

我认为你想反转那里的两个函数调用。

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