title: cannot determine truth value of Relational when applying lamdbdify
body: <p>This is kind of continuation from <a href="https://stackoverflow.com/questions/42528763/call-lambdify-in-a-loop-avoid-explicitly-call">here</a>.</p>
  
  <p>I have this code:</p>
  
  <pre><code>import sympy
  import numpy as np
  from sympy.utilities.lambdify import lambdify
  
  arr = [np.array([ 1, 2,3 ,4]), np.array([ 5, 6, 7, 8])]
  a,b = sympy.symbols('a b')
  var = [a,b]
  expr = 'a+b'
  new_dict = dict(zip(var, arr))
  print(new_dict)
  </code></pre>
  
  <p>which gives me:</p>
  
  <p><code>{a: array([1, 2, 3, 4]), b: array([5, 6, 7, 8])}</code></p>
  
  <p>Then , I use lambdify:</p>
  
  <p><code>f = lambdify( var, expr, 'numpy')</code></p>
  
  <p>If I try to apply <code>f</code>:</p>
  
  <pre><code>print(f([v for _,v in sorted(new_dict.items())]))
  </code></pre>
  
  <p>I am receiving:</p>
  
  <pre><code>TypeError: cannot determine truth value of Relational
  </code></pre>
  
  <p>According to comments in previous question, I have to use the <code>default_key</code> but I am not sure how to properly use it.</p>
  
  <p>If I try something like:</p>
  
  <pre><code>n = sorted(new_dict.keys(),  key=sympy.default_sort_key)
  
  for k,v in zip(n,new_dict.values()):
      print(f(v))
  </code></pre>
  
  <p>I am receiving:</p>
  
  <pre><code>&lt;lambda&gt;() missing 1 required positional argument: '_Dummy_21'
  </code></pre>
  
  <p>when I call <code>print(f(v))</code></p>