best way to go about with a script that seems to need both bash and python functionality

Python gives you total control of all the input and output streams of all the subprocesses. It's a bit more verbose than Bash but I promise it's not missing any features.

A demo of some PIPE gymnastics in case it's helpful: Two subprocesses interacting with each other and Python code to glue them together.

The first subprocess is a bash script that echoes lines from stdin with -I prepended. Python forwards stdin directly to this subprocess, but it intercepts stdout.

Each of those outputs is passed as an argument to a new date process. The -I<format> prints the ISO timestamp to given precision date, hours, etc. Python again intercepts the output.

If the date process succeeded, it URL-escapes the formatted date and prints it. If it failed (due to invalid argument), it prints the error message unchanged.

Keyboard interrupt Ctrl+C works at any point. You can also send end-of-file Ctrl+D to end everything gracefully.

import sys
from subprocess import Popen, PIPE, run
from urllib.parse import quote

COMMAND = '''
while read -r line; do
    echo "-I$line"
done
'''

proc = Popen(
    ['bash', '-c', COMMAND],
    stdin=sys.stdin,
    stdout=PIPE,
    encoding='utf8',
)

for fmt in proc.stdout:
    result = run(
        ['date', fmt.strip()],
        capture_output=True,
        encoding='utf8',
    )
    if not result.returncode:
        print(quote(result.stdout.strip()))
    else:
        print(result.stderr.strip())

$ python script.py
date
2024-09-11
huor
date: invalid argument ‘huor’ for ‘--iso-8601’
Valid arguments are:
  - ‘hours’
  - ‘minutes’
  - ‘date’
  - ‘seconds’
  - ‘ns’
Try 'date --help' for more information.
hour
2024-09-11T22-04%3A00
^CTraceback (most recent call last):
  File "/home/allem/PycharmProjects/scratch/runner.py", line 20, in <module>
    for fmt in proc.stdout:
KeyboardInterrupt

Now obviously in practice this particular problem could be solved in a million easier ways - but I hope this convinces you that Python is not missing any features regarding piping data and handling interrupts. I haven't even touched import io or os.mkfifo which together generalize shell scripting concepts like process substitution and tee and similar.

If Ctrl+C doesn't work for some reason, then some component somewhere is improperly intercepting those interrupts. It's probably not a Python (or bash) issue.

That's up to what the python parent wants to do. You can detect particular prompts from stdout, and pass through user input when it happens on a case-by-case basis.

For a more generic approach, you'll have to use either threading or asyncio. With async, for instance, you can just wait on stdin and stdout on the subprocess in a loop.