title: change a form value before validation in Django form
body: <p>I have a django form and on my view function I do this :</p>
  
  <pre><code>search_packages_form = SearchPackagesForm( data = request.POST )
  </code></pre>
  
  <p>I would like to overwrite a form field called price which is decleared as such :</p>
  
  <pre><code>price = forms.ChoiceField( choices = PRICE_CHOICES, required = False,widget = forms.RadioSelect )
  </code></pre>
  
  <p>I would like to overwrite the form field before calling <code>search_packages_form.is_valid()</code></p>
  
  <p>I thought of doing : </p>
  
  <pre><code>search_packages_form.data['price'] = NEW_PRICE
  </code></pre>
  
  <p>But it does not work. Any ideas ?</p>