title: Getting my formset to validate on the number of valid filled in forms
body: <p>I want the user to fill in exactly 3 forms in my formset, there are of course only 3 forms in my formset on the frontend but I want to make sure a user doesn't craft it's own POST. I've come up with this:</p>
  
  <pre><code>class ThisFormset(BaseInlineFormSet):
      def clean(self):
          results = super(ThisFormset, self).clean()
          count = len(filter(lambda f: f.is_valid() and f.clean(), self.forms))
  
          if count != 3:
              raise forms.ValidationError("Must fill in 3")
  
          return results
  </code></pre>
  
  <p>Is there a better way of doing the <code>len(filter(lambda f: f.is_valid() and f.clean(), self.forms))</code> portion to get the count?</p>