title: Dynamic choices for Django SelectMultiple Widget
body: <p>I'm building a form (not modelForm) where i'd like to use the SelectMultiple Widget to display choices based on a query done during the init of the form.</p>
  
  <p>I can think of a few way to do this but I am not exactly clear on the right way to do it. I see different options.</p>
  
  <p>I get the "choices" I should pass to the widget in the form <strong>init</strong> but I am not sure how I should pass them.</p>
  
  <pre><code>class NavigatorExportForm(forms.Form):
  
  def __init__(self,user, app_id, *args,**kwargs):
      super (NavigatorExportForm,self ).__init__(*args,**kwargs) # populates the form
      language_choices = Navigator.admin_objects.get(id=app_id).languages.all().values_list('language', flat=True)
  
  languages = forms.CharField(max_length=2, widget=forms.SelectMultiple(choices=???language_choices))
  </code></pre>