Line-by-line processing when stdin is not a tty

Cameron Simpson cs at
Wed Aug 11 11:42:30 CEST 2010

On 11Aug2010 00:11, RG <rNOSPAMon at> wrote:
| When stdin is not a tty, Python seems to buffer all the input through 
| EOF before processing any of it:
| [ron at mickey:~]$ cat | python
| print 123
| print 456 <hit ctrl-D here>
| 123
| 456
| Is there a way to get Python to process input line-by-line the way it 
| does when stdin is a TTY even when stdin is not a TTY?

What you're seeing here is not python's behaviour but cat's behaviour.

Almost all programs do line buffering (flush buffer at newline) when the
file is a terminal (character device) and block buffering (flush when a
fixed size buffer, typically 8192 bytes or some larger power of 2) when
the file is not a terminal. This is default behaviour for the stdio

So "cat" is simply not feeding any data to python until it has a lot of
it; there is nothing python can do about that. We would need to know
more about your specific task to suggest workarounds.

Usually you either
need an option on the upstream program to tell it to line buffer
explicitly or you need to play silly games with pseudo terminals to
convince the upstream program it is attached to a terminal. The latter
is both ugly and generally inadvisable because many programs that change
their buffering when attached to a terminal also change other behaviour,
such as issuing interactiove prompts etc.

Cameron Simpson <cs at> DoD#743

The type syntax for C is essentially unparsable. - Rob Pike

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