title: Python partial derivatives easy
body: <p>I'm interested in computing partial derivatives in Python. I've seen functions which compute derivatives for single variable functions, but not others.</p>
  
  <p>It would be great to find something that did the following</p>
  
  <pre><code>    f(x,y,z) = 4xy + xsin(z)+ x^3 + z^8y
      part_deriv(function = f, variable = x)
      output = 4y + sin(z) +3x^2
  </code></pre>
  
  <p>Has anyone seen anything like this?</p>