Select in Python

[Dio]

#!/usr/bin/env python2

from sys import stdin
from select import select

while 1:
(rr, wr, er) = select([stdin], [], [])
for fd in rr:
print fd

program block in the first select(), after I type something and "enter
", it never block in select() again,why?

[Fredrik Lundh]

Re: Select in Python

Dio wrote:
[color=blue]
> from sys import stdin
> from select import select
>
> while 1:
> (rr, wr, er) = select([stdin], [], [])
> for fd in rr:
> print fd
>
> program block in the first select(), after I type something and "enter
> ", it never block in select() again,why?[/color]

if there's data waiting to be read from the stream, select() will tell
you so every time you ask.

</F>

[K.S.Sreeram]

Re: Select in Python

Dio wrote:[color=blue]
> while 1:
> (rr, wr, er) = select([stdin], [], [])
> for fd in rr:
> print fd
>
> program block in the first select(), after I type something and "enter
> ", it never block in select() again,why?[/color]

select blocks until there is some data to read from stdin, but it does
not *clear* the data. so once you enter data, select will not block
until you read the data. once you're done reading the available data,
you can again use select to block until more data is available.

Regards
Sreeram



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[Gary Herron]

Re: Select in Python

Dio wrote:[color=blue]
> #!/usr/bin/env python2
>
> from sys import stdin
> from select import select
>
> while 1:
> (rr, wr, er) = select([stdin], [], [])
> for fd in rr:
> print fd
>
> program block in the first select(), after I type something and "enter
> ", it never block in select() again,why?
>
>[/color]
Because select blocks until there is something to read on stdin, and as
long as there is something to be read on stdin, it will not block. Since
your program never reads in the data on stdin, your "enter\n" just sits
there waiting to be read, and select always inform you of that fact by
returning stdin in the rr list.

If you want select to start blocking again, you must read all bytes from
stdin whenever select says there are bytes to be read.

Gary Herron

[Dio]

Re: Select in Python


K.S.Sreeram 写道：
[color=blue]
> Dio wrote:[color=green]
> > while 1:
> > (rr, wr, er) = select([stdin], [], [])
> > for fd in rr:
> > print fd
> >
> > program block in the first select(), after I type something and "enter
> > ", it never block in select() again,why?[/color]
>
> select blocks until there is some data to read from stdin, but it does
> not *clear* the data. so once you enter data, select will not block
> until you read the data. once you're done reading the available data,
> you can again use select to block until more data is available.
>
> Regards
> Sreeram[/color]

Thanks a lot！I use stdin.readline( ) after select and it's ok :)

[Lawrence D'Oliveiro]

Re: Select in Python

In article <mailman.7484.1 151389154.27775 .python-list@python.org >,
Gary Herron <gherron@island training.com> wrote:
[color=blue]
>If you want select to start blocking again, you must read all bytes from
>stdin whenever select says there are bytes to be read.[/color]

To add to this, it is a good idea to read the select_tut(2) man page,
particularly the "SELECT LAW" section. There's a lot of dos and don'ts
there on how to avoid common bugs.