autopage

Autopage is a Python library to
automatically display terminal output from a program in a pager (like less)
whenever you need it, and never when you don't. And it only takes one line of
code.

You know how some CLI programs like git (and a handful of others, including
man and systemctl) automatically pipe their output to less? Except not if
there's less than one screen's worth of data. And if you redirect the output to
a file or a pipe, it does the right thing instead. Colours are preserved. Don't
you wish all programs worked like that? Now at least all of your Python
programs can.

License

© 2020-2022 by Zane Bitter

Open Source licensed under the terms of the Apache Software License, version
2.0.

Installation

Autopage is available from PyPI. The
easiest way to install (preferably in a virtualenv virtual environment) is
with pip:

$ pip install autopage

On Fedora and CentOS/RHEL

Autopage is packaged for
Fedora
35 and later. To install:

# dnf install python3-autopage

Copr repositories are available for older versions of Fedora and EPEL. Before
attempting to install on those versions, first enable the copr repository:

# dnf copr enable zaneb/autopage

On Ubuntu and Debian

Autopage is packaged for
Ubuntu
jammy and later, and for Debian
bookworm. To install:

# apt-get install python3-autopage

A PPA is available for older versions of Ubuntu. Before attempting to install
on those versions, first enable the PPA:

# add-apt-repository ppa:zaneb/autopage
# apt-get update

On Gentoo

Autopage is packaged for
Gentoo. To install:

# emerge dev-python/autopage

Basic Use

The AutoPager class provides a context manager that furnishes the output
stream to write to. Here is a basic example that reads from stdin and outputs
to a pager connected to stdout:

import sys
import autopage

with autopage.AutoPager() as out:
    for l in sys.stdin:
        out.write(l)

If you are explicitly passing a stream to write to (rather than directly
referencing a global variable such as sys.stdout then you may be able to add
automatic paging support with only a single line of code.

Paging help output

If your program uses the argparse module from the standard library, you can
ensure that the help output is automatically paged when possible by changing
the import statement to:

from autopage import argparse

If you don't control the module that imports argparse, you can instead call
autopage.argparse.monkey_patch() to patch the module directly. This function
can also be used as a context manager.

Environment

The default pager command (autopage.command.DefaultPager()) allows the end
user to override the pager command by setting the PAGER environment variable.
To disable this behaviour, pass
pager_command=autopage.command.PlatformPager() to use the default pager for
the current platform, or pass a specific pager from autopage.command. The
default pager command is less on most platforms. On AIX the default pager
command is more, and on Windows more.com.

The end user can also override the settings for less by setting the LESS
environment variable. If not specified, the settings are determined by the
allow_color and line_buffering options. By default ANSI control characters
for setting colours are respected and the pager will not run if there is less
than a full screen of text to display.

Line buffering

Normally output streams are buffered so that data is written to the output file
only when the buffer becomes full. This is efficient and generally works fine
as long as the data is being produced as fast as it can be consumed. However,
when the data is streaming at a slower rate than it could be displayed (e.g.
log output from something like tail -f) this results in a large delay between
data being produced and consumed. If you have ever tried to grep a streaming
log and pipe the output to a pager then you are familiar with how
unsatisfactory this is.

The solution is to flush the output buffer after each line is written, which is
known as line
buffering. The
AutoPager class supports a line_buffering argument to enable or disable
line buffering. The default is to use the line buffering mode already
configured for the output stream (which is usually to disable line buffering).

When reading from an input stream (which may be a file, pipe, or the console)
and optionally processing the data before outputting it again, the convenience
function line_buffer_from_input() returns the optimal line buffering setting
for a given input stream (sys.stdin by default).

import sys
import autopage

with autopage.AutoPager(line_buffering=autopage.line_buffer_from_input()) as o:
    for l in sys.stdin:
        o.write(l)

Terminal reset

By default, when the pager exits it will leave the latest displayed output on
screen in the terminal. This can be changed by passing True for the
reset_on_exit argument to the AutoPager class. If this option is set, the
terminal will be cleared when the pager exits, returning to its position prior
to starting the pager (as is the case by default when running less manually
from the command line).

Exit code

Programs may wish to return a different exit code if they are interrupted by
the user (either with Ctrl-C or by closing the pager) than if they ran to
completion. The exceptions generated when the pager is closed prematurely are
suppressed, so the AutoPager class offers the exit_code() method to provide
a suitable exit code for the program. This also takes into account other
exceptions that bubble up through the context manager.

Complete Example

import sys
import autopage

def process(input_stream, output_stream):
    pager = autopage.AutoPager(
        output_stream,
        line_buffering=autopage.line_buffer_from_input(input_stream),
        allow_color=True,
        reset_on_exit=True,
        errors=autopage.ErrorStrategy.REPLACE,
    )

    try:
        with pager as out:
            for l in input_stream:
                out.write(l)
    except Exception as exc:
        sys.stderr.write(f'{str(exc)}\n')
    except KeyboardInterrupt:
        pass
    return pager.exit_code()

sys.exit(process(sys.stdin, sys.stdout))

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