是什么导致此错误？ 'Mul'对象没有属性'log'

Question

import numpy as np

import sympy as sym

r = sym.Symbol('r')

rhos =0.02*10**7

rs = 89.74

f = np.log(rhos/ ((r/rs)*(1+(r/rs))))

alpha = sym.diff(f,r)

---

AttributeError: 'Mul' object has no attribute 'log'