code: def get_only(self, product_type, target):
      
      product_mapping = self.get(product_type).get(target)
      if len(product_mapping) != 1:
        raise ProductError('{} directories in product mapping: requires exactly 1.'
                           .format(len(product_mapping)))
  
      for _, files in product_mapping.items():
        if len(files) != 1:
          raise ProductError('{} files in target directory: requires exactly 1.'
                             .format(len(files)))
  
        return files[0]
docs: If there is exactly one product for the given product type and target, returns the
      full filepath of said product.
  
      Otherwise, raises a ProductError.
  
      Useful for retrieving the filepath for the executable of a binary target.
  
      :API: public