How do I graph an input function?

Question

With pyplot you can plot a function with

x = np.linspace(-10, 10, num=100) #start x, end x, number of points on line

fx = []
for i in range(len(x)):
  fx.append(-2*x[i]**3 - 35*x[i]**2 + 39*x[i] + 70) #creates function f(x)

plt.plot(x, fx) #plots function f(x)
plt.grid() #adds grid
plt.axvline() #adds line at x=0
plt.axhline() #adds line at y=0  
plt.show() #actually makes graph appear

this will plot the function f(x) = -2x³-35x²+39x+70, but how do you make it so it will graph a function that you input?

I've tried just feeding in a string that follows the same format as what it is normally with

foo = "4*x[i]**3"
fx.append(foo)

but that just gives me this