Return screenshot data from subprocess without residual files

Question

I have a use case, where I want to take screenshots via scrot without leaving behind residual files:

"""Takes screenshots."""

from pathlib import Path
from subprocess import check_call
from tempfile import TemporaryDirectory

from digsigclt.os.posix.common import SCROT
from digsigclt.types import Screenshot


__all__ = ['screenshot']


JPEG = 'image/jpeg'
FORMATS = {
    'jpe': JPEG,
    'jpeg': JPEG,
    'jpg': JPEG,
    'png': 'image/png',
    'gif': 'image/gif'
}


def screenshot(filetype: str = 'jpg', display: str = ':0',
               quality: int = None, multidisp: bool = False,
               pointer: bool = False) -> Screenshot:
    """Takes a screenshot."""

    try:
        content_type = FORMATS[filetype]
    except KeyError:
        raise ValueError('Invalid image file type.') from None

    command = [SCROT, '--silent', '--display', display]

    if quality is not None:
        command += ['--quality', str(quality)]

    if multidisp:
        command.append('--multidisp')

    if pointer:
        command.append('--pointer')

    with TemporaryDirectory() as tmpd:
        tmpfile = Path(tmpd).joinpath(f'digsigclt-screenshot.{filetype}')
        command.append(str(tmpfile))
        check_call(command)

        with tmpfile.open('rb') as file:
            return Screenshot(file.read(), content_type)

The code works as intended. What bothers me, however, is the workaround that I had to take using TemporaryDirectory. All tests using a ǸamedTemporaryFile directly failed due to the fact, that python opens the target file before the subprocess writes to it, so that a subsequent read() would return an empty bytes object. Is there a way to improve the intermediate file handling without the TemporaryDirectory workaround and without opening and closing the intermediate file more than once?

Answer 1

All tests using a NamedTemporaryFile directly failed

The semantics for NamedTemporaryFile are a bit involved; you might want to read the fine manual, especially the per-OS behavior around the delete flag.

Given that by default scrot won't write to a file that exists, you may wish to unlink() prior to forking off a child.

annotation

def screenshot( ... ,
               quality: int = None, ...

Please prefer quality: int | None = None for this optional parameter.