title: Sympy function evaluation
body: <p>I have a little question about sympy.
  I did load the library with :</p>
  
  <pre><code>from sympy import *
  </code></pre>
  
  <p>At some point of my program I would like to evaluate a function. </p>
  
  <pre><code>x=Symbol('x', real=True)
  sqrt(1-x).subs(x, 9).evalf()
  &gt;&gt;&gt; 2.82842712474619*I
  </code></pre>
  
  <p>Sympy answer me complex value but i would like an error as in basic python : </p>
  
  <pre><code>sqrt(-1)
  &gt;&gt;&gt; ValueError: math domain error
  </code></pre>
  
  <p>Someone know how to do that using sympy?</p>