due to educational purposes I have some scenarios to deal with.
The main one is to simulate a noisy serial line using socat (I currently have version 1.7.4)

I’ll elaborate:
I have software that has to do with protocols over serial. I need to test its reliability and performance against errors and communication problems, in short against noisy channels.
With socat I can create a “clean” virtual channel:

socat -d -d -v -x pty,raw,echo=0,link=/tmp/ttyV1 pty,raw,echo=0,link=/tmp/ttyV2

I’m looking for a way for the data to be “corrupted” in the way between ttyV1 and ttyV2 (and vice versa).
I was thinking of a python script that somehow receives the data sent to ttyV1, processes it, and then transmits it to ttyV2.
I tried it by running 3 instances of socat:

• socatX: is what creates pty with socat -d -d -v -x pty,raw,echo=0,link=/tmp/ttyV1 pty,raw,echo=0,link=/tmp/ttyV2
• socat1: is socat -d -d -v -x /tmp/ttyV1,raw,echo=0 EXEC: "python3 disturb_script.py",pty,raw,echo=0
• socat2: is socat -d -d -v -x /tmp/ttyV2,raw,echo=0 EXEC: "python3 disturb_script.py",pty,raw,echo=0

in this case by sending data to /tmp/ttyV1, for example with cat >/tmp/ttyV1, I see data flowing without modification. I see the data trace (thanks to the -x -v parameters of socat) only in the socatX session.
I tried terminating socat1 and socat2, without touching socatX, and the data continues to flow. So I thought that sending the data to /tmp/ttyV1 only involves socatX.

then I changed something:

• socat1: create the first pty with socat -d -d -v -x pty,raw,echo=0,link=/tmp/ttyV1 EXEC: "python3 disturb_script.py",pty,raw,echo=0
• socat2: create the second pty with socat -d -d -v -x pty,raw,echo=0,link=/tmp/ttyV2 EXEC: "python3 disturb_script.py",pty,raw,echo=0
• socatX: I would like it to link the two pty using socat -d -d -v -x /tmp/ttyV1,raw,echo=0 /tmp/ttyV2,raw,echo=0

in this case the data, always sent with cat >/tmp/ttyV1, does not pass on /tmp/ttyV2. And I can see the data traces only on socat1.

the python script is this:

import sys
import random
import os

def disturb(data):
    """ Disturb data by altering or deleting characters. """
    result = bytearray()
    for byte in data:
        if random.random() < 0.1:  # 10% chance to change a bit in a byte
            byte ^= 1 << random.randint(0, 7)
        if random.random() > 0.05:  # 95% chance to keep the byte
            result.append(byte)
    return bytes(result)

def main():
    while True:
        data = sys.stdin.buffer.read(128)
        if not data:
            break
        print(f"Received data: {data}")  # Debug print
        disturbed_data = disturb(data)
        print(f"Sending data: {disturbed_data}")  # Debug print
        sys.stdout.buffer.write(disturbed_data)
        sys.stdout.buffer.flush()

if __name__ == '__main__':
    main()

How can I get to my goal?

PS:
In the beginning I told you about other scenarios.
Another scenario is one where I have a true serial; I should create a virtual serial connected to it, and in between put a script that processes (encrypts/decrypts) the data