Example of how to exhaust stdin from a pipe and then call a function or launch an external program with stdin connected to the tty.

redirect.py

#!/usr/bin/env python3

# Example of how to exhaust stdin from a pipe and then call a function
# or launch an external program with stdin connected to the tty.

from contextlib import contextmanager
import os
import subprocess
import sys

@contextmanager
def redirect_stdin(fp):
    """Redirect stdin to a file like object or character device.

    This is a single use context manager.
    """
    saved_stdin = sys.stdin
    sys.stdin = fp
    yield
    sys.stdin = saved_stdin

def read_tty():
    """Read a line from tty and echo back what is read."""
    sys.stderr.write("Please enter something and press enter.\n")
    with open('/dev/tty', 'r') as devtty:
        with redirect_stdin(devtty):
            line = sys.stdin.readline()
            print(line)

def launch_editor():
    """Launch $EDITOR, or Emacs if $EDITOR is not available."""
    with open('/dev/tty', 'r') as devtty:
        editor = os.environ['EDITOR'] if 'EDITOR' in os.environ else 'emacs'
        subprocess.call([editor], stdin=devtty)

def main():
    """CLI."""
    # exhaust stdin first
    sys.stdin.read()
    # redirect stdin to tty in a function call
    read_tty()
    # redirect stdin to tty when launching an external program
    launch_editor()

if __name__ == '__main__':
    main()