read and readline hanging

[Marc 'BlackJack' Rintsch]

On Fri, 25 Jan 2008 17:31:16 +0100, Olivier Lefevre wrote:

> Thanks for the answer. Yes this is tricky. I have done it in Java
> before, where you can, e.g., set up a thread to pump stuff out of
> both stderr and stdout continuously but my python is too rudimentary
> for that.

The `trheading` module is modeled after Java's threading API.

> There is a key difference anyway: in Java you can write
> 
>      while (br.readLine() != null) { <pump> }
> 
> where br is the buffered reader in which you've wrapped the stdout
> of the child process and it will not hang. But in python eventually
> stdout.readline() hangs. This is a real nuisance: why can't it just
> return None?

Because that would be quite annoying because most of the time people want
blocking behavior.

>> 1. The subprocess has stopped producing output.
> 
> Indeed, if I do this interactively, I can tell after 3 lines that I've
> gotten all there is to get right now and the fourth readline() call
> hangs. But how can I find out *programmatically* that there is no more
> input?

You can't.

>> If you are only reading its standard output, are you sure that the 
>  > subprocess is flushing its buffers so you can recognize it's time to
>  > provide more input?
> 
> The subprocess in a purely passive position: it is an interpreter: I
> send it commands and I read the answers. The python side is in charge.

This doesn't answer if the interpreter doesn't flush its output buffer
after every line.

Ciao,
	Marc 'BlackJack' Rintsch