code: def resolve(self, _):
          
          if self.default_value == DUMMY_VALUE:
              if self.name in os.environ:
                  return os.environ[self.name]
              else:
                  raise VelException(f"Undefined environment variable: {self.name}")
          else:
              return os.environ.get(self.name, self.default_value)
docs: Resolve given variable