code: def set_v(self, v):
      if v is None:
          self._v = None
          return
      if not isinstance(v, Symbol):
          raise ValueError("v must be a SymPy Symbol.")
      self._v = v
  
imports: from sympy.core.singleton import S
  from sympy.core.symbol import Symbol
  from sympy.core.sympify import sympify
  from sympy.core.numbers import Integer
label_desc: Use image node_id 4 for calling the PlotInterval obj's underlying member method code with example usage: obj.set_v(v) without return types