title: cannot set initial value for forms.ChoiceField in django
body: <p>everyone</p>
  
  <p>I want to set initial value for ChoiceField while that value isn't in choices of ChoiceField</p>
  
  <p>following is the ArticleForm definition:</p>
  
  <pre><code>def ArticleForm(forms.Form):
    choice = forms.ChoiceField(choices=[['a':'a'],['b':'b']])
  </code></pre>
  
  <p>then I instantiated the form by passing the initial argument:</p>
  
  <pre><code>form = ArticleForm(initial={'choice':'Portrush'})
  </code></pre>
  
  <p>notice that the initial value('Portrush') isn't one of choices defined in ChoiceField('a' and 'b')</p>
  
  <p>How could I set initial value? Any suggestion is appreciated.</p>