Python os.fdopen() gives OSError: [Errno 9] Bad file descriptor

Question

I am trying to test out a virtual serial connection in python with individual funtions for each test.

'''
Testing Virtual Serial Port Connections
'''

import os
import sys
import time
import serial

DATA = b'Hello\\n'

class TestSerial():
"""Test PTY serial open"""

    def setUp(self):
        # Open PTY
        self.master, self.slave = os.openpty()
        self.s_name = os.ttyname(self.slave)
        self.ser = serial.Serial(self.s_name, timeout=1)
    
    def openPtySlave(self):
        with serial.Serial(os.ttyname(self.slave), timeout=1) as slave:
            pass  # OK
    
    def testPtyWrite(self):
        fd = os.fdopen(self.master,'wb') 
        fd.write(DATA)
        fd.flush()
        out = self.ser.read(len(DATA))
        print(out)
                
    
    def testPtyRead(self):
        fd = open(self.master, 'rb')
        self.ser.write(DATA)
        self.ser.flush()
        out = fd.read(len(DATA))
        print(out)

if __name__ == '__main__':
sys.stdout.write(__doc__)

    PtyTest = TestSerial()
    PtyTest.setUp()
    PtyTest.openPtySlave()
    PtyTest.testPtyWrite()
    PtyTest.testPtyRead()

But every time I execute the code I get the following error:

Traceback (most recent call last):
File "/Main/pty.py", line 48, in \<module\>
PtyTest.testPtyRead()
File "/Main/pty.py", line 35, in testPtyRead
fd = open(self.master, 'rb')
OSError: \[Errno 9\] Bad file descriptor

The script opens the file to write to it but fails in the read function.

I've tried closing the file in the write function before exiting but I still get the error.

Can anyone help?