python求导数入门例子

#!/usr/bin/python
#求导数

def func(coeff):
  sum=''
  for key in coeff:
    sum=sum+'+'+str(key)+'*'+'x'+'**'+str(coeff[key])
  return sum[1:]
 
from sympy import *
from sympy.core.sympify import SympifyError
expr = func({2:0,3:1,4:2,5:7})
x = Symbol("x")
sexpr = sympify(expr)
print diff(sexpr, x)
print diff(sexpr, x).subs('x',3)