title: Python How to get the value of one specific point of derivative?
body: <pre><code>from sympy import *
  x = Symbol('x')
  y = x ** 2
  dx = diff(y, x)
  </code></pre>
  
  <p>This code can get the derivative of y.
  It's easy <code>dx = 2 * x</code></p>
  
  <p>Now I want to get the value of <code>dx</code> for <code>x = 2</code>.</p>
  
  <p>Clearly, <code>dx = 2 * 2 = 4</code> when <code>x = 2</code></p>
  
  <p>But how can I realize this with python codes? </p>
  
  <p>Thanks for your help!</p>