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)

(2+3*x+4*x**2+7*x**3.7).diff(x).subs({x:3}).evalf()