title: How to subsitute symbols in sympyfied expression properly?
body: <p>my goal is to have a string turned into a symbolic expression using sympify and then make substitutions.</p>
  <pre><code>import sympy as sp
  
  Eq_Str = 'a*x+b'
  Eq_Sym = sp.sympify(Eq_Str)
  </code></pre>
  <p>Then, for instance, substitute <code>a</code> for something else:</p>
  <pre><code>Eq_Sym.subs(a,2)
  </code></pre>
  <p>But I get the error:</p>
  <pre><code>Traceback (most recent call last):
  
    File &quot;&lt;ipython-input-5-e9892d6ffa06&gt;&quot;, line 1, in &lt;module&gt;
      Eq_Sym.subs(a,2)
  
  NameError: name 'a' is not defined
  </code></pre>
  <p>I understand that there is no symbol <code>a</code> in the workspace. Am I right?
  Is there a way to get the symbols from the set I get from <code>Eq_Sym.free_symbols</code> into the workspace so I can substitute them in <code>Eq_Sym</code>.</p>
  <p>Thank you very much for taking the time to read this.</p>