title: Optional input data
body: <p>For the problem formulation </p>
  
  <pre class="lang-py prettyprint-override"><code>import pyomo.environ as pe
  
  model = pe.AbstractModel()
  model.I = pe.Set()
  model.p = model.Param(model.I)
  model.create_instance("input.dat")
  </code></pre>
  
  <p>and the <code>input.dat</code></p>
  
  <pre><code>set I := 1 2 3 ;
  param p :=
  1 0.1
  2 0.2
  3 0.3
  ;
  param q :=
  1 1.1
  2 2.2
  3 3.3
  ;
  </code></pre>
  
  <p>The following error is shown</p>
  
  <pre><code>AttributeError: 'AbstractModel' object has no attribute 'q'
  </code></pre>
  
  <p>How to silence <code>create_instance</code> in this case? The model is fully specified. The "excess" data (parameter q in this case) is needed for another model and the models share this input.dat. I could go with a try/except for the <code>AttributeError</code> and just carry on I guess, but then I would need to guard each <code>create_instance</code> call. I looked for a "skip_undefined" kwarg or similar in the documentation. Is there another preferred way to handle this situation?</p>