title: How do you make django form validation dynamic?
body: <p>I have a form that needs to have either a valid url or a valid file for uploading:</p>
  
  <pre><code>class ResourceUpload(ModelForm):
     ...        
     uploadedfile = forms.FileField('file')
     url_address = forms.URLField('url') 
     ...
  </code></pre>
  
  <p>How can I override the <code>FileField</code> and <code>URLField</code> validators so that Django only raises an error if both of the fields above are invalid but excepts one being invalid so long as the other is valid?</p>