title: Django Custom Form Validation not working
body: <p>I am trying to get some custom form validation working on Django but its not currently working.</p>
  
  <pre><code>class PostCodeForm (forms.Form):
      pcode = forms.CharField()
  
      def clean_pcode(self):
          permitted = {'a','b','c','d'}
          pcode = self.cleaned_data['pcode']
          if not str(permitted) in pcode:
              raise forms.ValidationError("Apologies, but surrey Spice does not currently deliver to you postcode.")
          return pcode
  </code></pre>
  
  <p>The end goal is that anything not in that tuple should not be permitted and should return the validation error. Any help is really appreciated.</p>