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A library to provide automatic paging for console output

Project description

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

© 2020-2021 by Zane Bitter

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

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.

Environment

The end user can override the pager command by setting the PAGER environment variable. The default command is less.

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=line_buffer_from_input()) as out:
    for l in sys.stdin:
        out.write(l)

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_buffered_input(input_stream),
        allow_color=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'{stf(exc)}\n')
    except KeyboardInterrupt:
        pass
    return pager.exit_code()

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

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