docs: def to_python(self, value):
          """Convert value if needed."""
          if isinstance(value, str):
              return value
          elif isinstance(value, dict):
              try:
                  value = value['url']
              except KeyError:
                  raise ValidationError("dictionary must contain an 'url' element")
  
              if not isinstance(value, str):
                  raise ValidationError("field's url element must be a string")
  
              return value
          elif not isinstance(value, None):
              raise ValidationError("field must be a string or a dict")