title: Convert a string to a SymPy expression?
body: <p>I'm trying to convert a string to a SymPy expression. I've tried <code>sympify()</code> and <code>parse_expr()</code> but both are returning errors. How can I correct these errors or workaround them using an alternative method? I'm using Python 3.4.</p>
  
  <pre><code>from sympy import *
  s = 'C+O*2'
  expr = sympify(s)
  
  from sympy.parsing.sympy_parser import parse_expr
  s = 'C+O*2'
  expr = parse_expr(s)
  </code></pre>
  
  <p>Both of these functions return the following error:</p>
  
  <pre><code>File "C:\...\sympy\core\mul.py", line 182, in flatten
      r, b = b.as_coeff_Mul()
  
  TypeError: as_coeff_Mul() missing 1 required positional argument: 'self'
  </code></pre>