[Python] When creating LangChain custom tool, how can I add new property

I want to create a custom tool class with an additional property, let's say number. How can I change this code so that it doesn't throw an error?

Code:

from langchain.tools import BaseTool


class MyTool(BaseTool):
name = "Whatever"
description = "Whatever"

def __init__(self):
self.number = 123

def _run(self):
pass

def _arun(self):
pass


tool = MyTool()


Error:

ValueError: "MyTool" object has no field "number"

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