clear doesn't take effect immediately on Linux

[belliriccardo]

Hi! I'm writing a small script and I'm using click for user input and console text formatting. In practice, I use the click.getchar() function to read a single char from stdin, and do some processing from there; I construct a string representing a visual representation of the output and I print it. But! Whenever I use click.clear() instead of os.system('clear'), I get an inconsistent behavior: the terminal doesn't get cleared at the beginning of the script,

Here is a minimal reproducible example (imports omitted for brevity): script1.py

def main():
    os.system('clear')
    c = click.getchar()

if __name__ == '__main__':
    main()

python3 script1.py -> the terminal gets cleared and there's only the blinking cursor; this is the expected behavior. Instead, here's script2.py:

def main():
    click.clear()
    c = click.getchar()

if __name__ == '__main__':
    main()

python3 script2.py -> see:

The terminal doesn't get cleared, I can still see my username, the command, etc. Obviously this is trivial to circumvent; my implementation is:

def clearscreen(use_click=False):
    click.clear() if use_click else os.system('cls' if os.name == 'nt' else 'clear')

Something like this could be mainlined in click.clear(), for example something like (taking from the current implementation):

def clear(scroll: bool = True) -> None:
    if not isatty(sys.stdout):
        return
    if WIN:
        os.system("cls")
    else:
        sys.stdout.write("\033[2J\033[1;1H") if scroll else os.system("clear")

Obviously the default value would follow the current behavior for consistency. I hope I've been thorough enough. Thanks!

• OS: Ubuntu 22.04
• Python version: 3.10.4
• Click version: 8.0.3

[davidism]

Using click.echo instead of sys.stdin.write ensures that the control code is flushed and takes effect before the next terminal interaction. Until a fix is released, you can manually do sys.stdout.flush() after click.clear().

Also, there's no need for the if WIN section anymore. Colorama is always required on Windows now, and it handles the ANSI clear all code.

[belliriccardo]

Yes I was thinking it had to do something with the flushing of the buffer. The last snippet of code I pasted was modified from click.clear(), maybe it can be rearranged with your suggestions like:

def clear() -> None:
    if not isatty(sys.stdout):
        return
    echo("\033[2J\033[1;1H", nl=False)

What do you think?

[davidism]

Yes, I tried that earlier on Linux and Windows and it worked. You're welcome to make a PR, or I will later.

[belliriccardo]

If it's not urgent I'll do it in a few hours (tomorrow)

[belliriccardo]

PR #2286 should be good to go! Sorry I made another one and then closed it, it's my first PR to a public repo, I wanted to update it again to let it pass all the checks.