Question

import sympy
from sympy import *
from sympy import diff
import numpy as np

alpha = Symbol('alpha')
beta = Symbol('beta')
delta = Symbol('delta')
css = Symbol('css')
cssn = Symbol('cssn')
kssn = Symbol('kssn')

kssn = ((((1/beta)+ (alpha*delta) - 1)/(1-alpha)) + delta)**alpha
css = (((1/beta)+ (alpha*delta) - 1)/(1-alpha))*kssn
cssn = (((1/beta)+ (alpha*delta) - 1)/(1-alpha))*kssn

eps = 0.0001*np.abs(css)
eps1 = 0.0001*np.abs(kssn)

res = (1/css) - (beta/cssn)*(1 - delta + (1 - alpha)*(kssn**(-alpha)))
res1 = (1/css+eps) - (beta/cssn)*(1 - delta + (1 - alpha)*(kssn**(-alpha)))
res2 = (1/css) - (beta/cssn+eps)*(1 - delta + (1 - alpha)*(kssn**(-alpha)))
res3 = (1/css) - (beta/cssn)*(1 - delta + (1 - alpha)*((kssn+eps1)**(-alpha)))

x1 = diff(res1, css)
x2 = diff(res2, cssn)
x3 = diff(res3, kssn)

b1 = (x1 - x)/eps
b2 = (x2 - x1)/eps
b3 = (x3 - x2)/eps1

错误讯息：

  File "hw5.py", line 33, in <module>
    x1 = diff(res1, css)
  File "/Users/obrienstp/anaconda/lib/python2.7/site-packages/sympy/core/function.py", line 1641, in diff
    return Derivative(f, *symbols, **kwargs)
  File "/Users/obrienstp/anaconda/lib/python2.7/site-packages/sympy/core/function.py", line 1029, in __new__
    Can\'t differentiate wrt the variable: %s, %s''' % (v, count)))
ValueError: 
Can't differentiate wrt the variable: 0.0103498953698543, 1

有人会帮我解决这个问题吗？我真的要在墙上打一个洞。我是编程新手，花了3天时间研究这个问题。为什么要说这个错误，我该如何解决呢？

Answer 1

当你遇到这样的情况时，请使用口译员给你反馈：

>>> # what variables are in res1?
...
>>> res1.free_symbols
set([delta, alpha, beta])

>>> # what is css?
...
>>> css
(delta + (alpha*delta - 1 + 1/beta)/(-alpha + 1))**alpha*(alpha*delta - 1 + 1/be
ta)/(-alpha + 1)

如前所述，你无法区分表达方式。您只能区分符号（或函数）。

Answer 2

问题是你第一次有

css = Symbol('css')
cssn = Symbol('cssn')
kssn = Symbol('kssn')

这将Python变量css，cssn和kssn定义为SymPy符号css，cssn和kssn。你有

kssn = ((((1/beta)+ (alpha*delta) - 1)/(1-alpha)) + delta)**alpha
css = (((1/beta)+ (alpha*delta) - 1)/(1-alpha))*kssn
cssn = (((1/beta)+ (alpha*delta) - 1)/(1-alpha))*kssn

这会覆盖Python变量以引用给定的表达式。我不能告诉你必须解决这个问题，因为我不确定你打算做什么。例如，当您撰写res = (1/css) - (beta/cssn)*(1 - delta + (1 - alpha)*(kssn**(-alpha)))时，您希望该表达式中的css为Symbol('css')还是(((1/beta)+ (alpha*delta) - 1)/(1-alpha))*kssn？

我认为你对Python变量和SymPy符号的工作原理感到困惑。试着直截了当，你将避免在将来犯同样的错误。有关此问题的更多信息，请访问http://docs.sympy.org/latest/tutorial/gotchas.html#symbols。

正如@smichr所说，在ipython，isympy或IPython笔记本等交互式终端中处理事情会让事情更加清晰，因为您可以立即看到变量实际上等于。

无法区分python中的变量

2 个答案: