title: apply changes to running instance of a class from another class
body: <p>Some weeks ago I had to simplify a program drastically, because I couldn't find a solution for this probably simple problem.
  Now I'm stuck at the same issue again and hope somebody can help me to fix it.</p>
  <p>I'm using PyQt5 and started a <code>QWidget</code> out of my main class <code>Class_One</code>, from which I opened a second class <code>Class_Two</code> with the following lines.</p>
  <pre><code>def on_create_search(self):
      self.search_form = Class_Two()
      self.search_form.show() 
  </code></pre>
  <p>In <code>Class_Two</code> I save changes to a JSON file, which should also effect a dictionary and a visible <code>QTreeWidget</code> from <code>Class_One</code>.</p>
  <p>So what I want to do is, to execute a method in <code>Class_One</code>, triggered by <code>Class_Two</code>, just as it was called from the running instance of <code>Class_One</code>.
  The called method should read in the JSON file and apply the changes to the data and the GUI of the running instance of <code>Class_One</code>.</p>
  <p>Calling such a method from another instance of <code>Class_One</code> didn't help me out, because the changes have to be applied to my running class instance of <code>Class_One</code>.
  Perhaps there is an even better way, so I'm open for any suggestions.</p>