title: Sympy calculus: when evaluating a sympy expression, python outputs log(2), and e^0.3, as symbols, instead of computing their value
body: <p>So I have to write a python script for finding the root of a function using Newton's Method, and my code isn't computing a float when evaluating the derivative at a point, it's only evaluating the sin(x) part, leaving e^x and log(2) as they are.
  I'm stuck, please help.
  This is my code so far:</p>
  <pre><code>from sympy import *
  
  x=symbols('x')
  
  f=&quot;(e**x)+(2**(-x))+2*cos(x) - 6&quot;
  func=sympify(f)
  fdiff=Derivative(func, x) #this returns an expression for the derivative
  val=fdiff.doit().subs({x: 0.4})
  print(val) #this should output 0.187681....  instead it returns another expression
  </code></pre>
  <p>PS: evalf doesn't work either, neither does subs(x, 0.4)</p>