code: def convert_value(self, value):
          
          if isinstance(value, str):
              assert value.startswith(self.symbol), "Currency does not start with " + self.symbol
              value = value.lstrip(self.symbol)
          return super().convert_value(value)
docs: Convert value to float. If value is a string, ensure that the first
          character is the same as symbol ie. the value is in the currency this
          formatter is representing.