title: Change Django Form Data before it's processed
body: <p>I have a form for a model which has two fields: field_A and field_B. I want to:</p>
  <ul>
  <li>when the form is rendered: only field_A is displayed</li>
  <li>when I call form.save() field_B is saved in the model with the value from field_A</li>
  </ul>
  <p>What I've tried:</p>
  <pre><code>field_A = forms.CharField(required=True)
  field_B = forms.CharField(required=False)
  
  def save(self, *args, **kwargs):
       &quot;&quot;&quot;
       Overriding save, so call the parent form save and return the new_user
       object.
       &quot;&quot;&quot;
       self.data[&quot;field_B&quot;] = self.data[&quot;field_A&quot;]
       self.cleaned_data[&quot;username&quot;] = self.cleaned_data[&quot;email&quot;]
       super(MyParentClass*, self).save(*args, **kwargs) 
  </code></pre>
  <p>*both fields are inherited from ParentClass, where they are required</p>