auto_pager.py

#!/usr/bin/env python3

# auto_pager.py
# Copyright (C) 2018 Ryan Finnie
#
# This program is free software; you can redistribute it and/or
# modify it under the terms of the GNU General Public License
# as published by the Free Software Foundation; either version 2
# of the License, or (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
# 02110-1301, USA.

import sys
import os
import shlex
import subprocess


class AutoPager:
    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        self.close()

    def __init__(self):
        self.closed = False
        self.pager = None
        if sys.stdout.isatty():
            pager_cmd = ["pager"]
            if os.environ.get("PAGER"):
                pager_cmd = shlex.split(os.environ.get("PAGER"))
            env = os.environ.copy()
            if not os.environ.get("LESS"):
                env.update({"LESS": "-FRSXMQ"})
            try:
                self.pager = subprocess.Popen(
                    pager_cmd,
                    stdin=subprocess.PIPE,
                    stdout=sys.stdout,
                    encoding="UTF-8",
                    env=env,
                )
            except FileNotFoundError:
                pass

    def write(self, l):
        if self.closed:
            return

        if self.pager:
            try:
                self.pager.stdin.write(l)
            except KeyboardInterrupt:
                self.close()
            except BrokenPipeError:
                self.close()
        else:
            try:
                sys.stdout.write(l)
            except BrokenPipeError:
                self.close()

    def close(self):
        if self.closed:
            return

        if self.pager:
            try:
                self.pager.stdin.close()
            except BrokenPipeError:
                pass
            ret = None
            while ret is None:
                try:
                    ret = self.pager.wait()
                except KeyboardInterrupt:
                    continue

        self.closed = True


if __name__ == "__main__":
    with AutoPager() as pager:
        for i in range(1000):
            print(i, file=pager)