我一直得到一个“TypeError：-：'NoneType‘和'NoneType’的不支持的操作数类型”

Question

我正在学习python，并且作为实践，我编写了一些代码来查找用户定义函数的派生函数。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)

完整的错误代码如下

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

您忘记了从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，如果您试图除以2h，您将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)

我想你想把这两个函数的调用倒过来。

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

Answer 2

您的derivative_formula没有在python中默认为return None的return语句。因此，当您调用abs(derivative_estimate - V)时，您实际上是从另一个NoneType对象中减去一个NoneType对象(V)。

您需要的是derivative_formula中的返回语句

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