Why can't add file handler with the form of self.fh in the init method?

os and python info:

uname -a
Linux debian 5.10.0-8-amd64 #1 SMP Debian 5.10.46-4 (2021-08-03) x86_64 GNU/Linux
python3 --version
Python 3.9.2

Here is a simple class which can start multiprocessing.

from multiprocessing.pool import Pool    

class my_mp(object):
    def __init__(self):
        self.process_num = 3
        fh = open('test.txt', 'w')
    def run_task(self,i):
        print('process {} start'.format(str(i)))
        time.sleep(2)
        print('process {} end'.format(str(i)))
    def run(self):
        pool = Pool(processes = self.process_num)
        for i in range(self.process_num):
            pool.apply_async(self.run_task,args = (i,))
        pool.close()
        pool.join()

Initialize the my_mp class,then start multiprocess.

ins = my_mp()
ins.run()
process 0 start
process 1 start
process 2 start
process 0 end
process 2 end
process 1 end

Now replace fh = open('test.txt', 'w') with self.fh = open('test.txt', 'w') in my_mp class and try again.

ins = my_mp()
ins.run()    

No output!Why no process start?

>>> from multiprocessing.pool import Pool    
>>> 
>>> class my_mp(object):
...     def __init__(self):
...         self.process_num = 3
...         fh = open('test.txt', 'w')
...     def run_task(self,i):
...         print('process {} start'.format(str(i)))
...         time.sleep(2)
...         print('process {} end'.format(str(i)))
...     def run(self):
...         pool = Pool(processes = self.process_num)
...         for i in range(self.process_num):
...             pool.apply_async(self.run_task,args = (i,))
...         pool.close()
...         pool.join()
... 
>>> x = my_mp()
>>> x.run()
process 0 start
process 1 start
process 2 start
process 2 end
process 0 end
process 1 end
>>> class my_mp(object):
...     def __init__(self):
...         self.process_num = 3
...         self.fh = open('test.txt', 'w')
...     def run_task(self,i):
...         print('process {} start'.format(str(i)))
...         time.sleep(2)
...         print('process {} end'.format(str(i)))
...     def run(self):
...         pool = Pool(processes = self.process_num)
...         for i in range(self.process_num):
...             pool.apply_async(self.run_task,args = (i,))
...         pool.close()
...         pool.join()
... 
>>> x = my_mp()
>>> x.run()
>>> x.run()
>>> x = my_mp()
>>> class my_mp(object):
...     def __init__(self):
...         self.process_num = 3
...         fh = open('test.txt', 'w')
...         self.fh = fh
...     def run_task(self,i):
...         print('process {} start'.format(str(i)))
...         time.sleep(2)
...         print('process {} end'.format(str(i)))
...     def run(self):
...         pool = Pool(processes = self.process_num)
...         for i in range(self.process_num):
...             pool.apply_async(self.run_task,args = (i,))
...         pool.close()
...         pool.join()
... 
>>> x = my_mp()
>>> x.run()
>>> 

Why can't add file handler with the form of self.fh in the __init__ method?I have never called the file handler defined in __init__ in any process.

from Why can't add file handler with the form of self.fh in the init method?