Question

我不能lambdify包含衍生物的表达式。以下是解释问题的最低代码：

import sympy
x = sympy.symbols('x', real=True)
a = sympy.Function('a')(x)
b = a**2
r = sympy.diff(b, x)
sympy.lambdify((a), b)  # works
r.evalf(subs={diff(a, x): 1, a: 2})  # works 
b_x = sympy.lambdify((diff(a, x), a), r)  # throws an error

最后一行代码会引发以下错误：

Traceback (most recent call last):   File "C:\Anaconda2\lib\site-packages\IPython\core\interactiveshell.py", line 2882, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)   File "<ipython-input-29-a0069a059795>", line 2, in <module>
    gx = sympy.lambdify((diff(a, x), a), r)  # gives an error   File "C:\Anaconda2\lib\site-packages\sympy\utilities\lambdify.py", line 434, in lambdify
    func = eval(lstr, namespace)   File "<string>", line 1
    lambda Derivative(a(x), x),_Dummy_29: (2*Derivative(_Dummy_29, x)*_Dummy_29)
                     ^ SyntaxError: invalid syntax

Answer 1

lambdify尝试用数字等价物替换符号函数返回一个可以接受数值或数组并返回数字或数组的函数。

错误消息

lambda Derivative(a(x), x),_Dummy_29: (2*Derivative(_Dummy_29, x)*_Dummy_29)
                     ^ SyntaxError: invalid syntax

表明lambdify正在尝试构建一个lambda函数，其第一个参数是Derivative(a(x), x)。显然，这不起作用。我们需要一个裸变量名而不是符号表达式Derivative(a(x), x)。

因此，请使用空符号Derivative(a(x), x)替换da：

import sympy
x = sympy.symbols('x', real=True)
a = sympy.Function('a')(x)
b = a**2
r = sympy.diff(b, x)
da = sympy.symbols('da')
b_x = sympy.lambdify((da, a), r.subs([(sympy.diff(a, x), da)]))
assert b_x(1, 2) == r.evalf(subs={sympy.diff(a, x): 1, a: 2})
# 4

不能用衍生物表达同情表达

1 个答案: