title: Automatic display of TCL sub field in TK label
body: <p>I have a variable and a label that shows the content of the variable. And it changes every time the variable has a new value:</p>
  
  <pre><code>global MyVariable
  label .mylabel -textvariable MyVariable
  </code></pre>
  
  <p>That works. </p>
  
  <p>Now the problem is that <code>MyVariable</code> does not contain the value that I want to show but it is a list of values and I want to execute some code that extracts the sub field that actually contains the data to be displayed. Is there an easy way to put some glue logic into that label in order to do preprocessing on the changed variable content before it gets displayed?</p>