Why when I write to stderr I see STDERR output on the stdout?

Question

import sys

if __name__ == '__main__':
    sys.stdout.write("STDOUT\n")
    sys.stderr.write("STDERR\n")

Could you explain why when I write to stderr I see STDERR output on the stdout? I supposed only STDOUT should be visible in the terminal.

$ python stdout_stdin.py
STDOUT
STDERR

Answer 1

I supposed only STDOUT should be visible in the terminal.

Incorrect. stderr is a separate file descriptor, but it's still connected to the same tty as stdout by default.