Reading from stdin

[Jean-Paul Calderone]

On Wed, 8 Oct 2008 09:28:55 -0400, Luis Zarrabeitia <kyrie at uh.cu> wrote:
>On Tuesday 07 October 2008 11:27:19 pm George Sakkis wrote:
>> """
>> In order to make a for loop the most efficient way of looping over the
>> lines of a file (a very common operation), the next() method uses a
>> hidden read-ahead buffer. As a consequence of using a read-ahead
>> buffer, combining next() with other file methods (like readline())
>> does not work right.
>> """
>>
>> I guess the phrasing "hidden read-ahead buffer" implies that buffering
>> cannot be turned off (or at least it is not intended to even if it's
>> somehow possible).
>
>Hmm. I wonder how those optimizations look like. Apparently, readline() cannot
>read from that read-ahead buffer, and that by itself sounds bad. Currently,
>if you loop a few times with next, you cannot use readline afterwards until
>you seek() to an absolute position.
>
>Actually, I think I may be replying to myself here. I imagine that 'next' will
>read a block instead of a character, and look for lines in there, and as the
>underlying OS likely blocks until the whole block is read, 'next' cannot
>avoid it. That doesn't explain, though, why readline() can't use next's
>buffer, why next doesn't have a sensible timeout for interactive sessions
>(unless the OS doesn't support it), and why the readahead cannot be turned
>off.
>
>I think I'll have to stick for now with the iter(function,sentinel) solution.
>
>And I may try to find next()'s implementation... I guess I'll be downloading
>python's source when my bandwidth allows it (or find it on a browseable
>repository)
>
>On a related note, help(file.read) shows:
>
>=====
>read(...)
>    read([size]) -> read at most size bytes, returned as a string.
>
>    If the size argument is negative or omitted, read until EOF is reached.
>    Notice that when in non-blocking mode, less data than what was requested
>    may be returned, even if no size parameter was given.
>=====
>
>But it doesn't say how to put the file object in non-blocking mode. (I was
>trying to put the file object in non-blocking mode to test next()'s
>behavior). ??Ideas?
>

Take a look at http://twistedmatrix.com/trac/browser/trunk/twisted/internet/fdesc.py

Jean-Paul