[Python-Dev] fileobj.read(float): warning or error? (original) (raw)

Cameron Simpson cs at zip.com.au
Wed Jul 23 00:46:29 CEST 2008

On 21Jul2008 23:35, Leif Walsh <leif.walsh at gmail.com> wrote: | On Tue, 22 Jul 2008, Cameron Simpson wrote: | > Leaving aside the 0.2 => 0 converstion, shouldn't read() raise an | > exception if asked for < 1 bytes? Or is there a legitimate use for_ | _> read(0) with which I was not previously aware? || I think read(0) should be a no-op, just like it is in libc. This lets | you write 'read(bytes)' without worrying about checking bytes, and | also lets you silently stop reading when you have no more space, like | in the following: || buf = f.read(max(bytesleft, pagesize)) | while buf: | process(buf) # updates bytesleft | buf = f.read(max(bytesleft, pagesize))

[ Don't you mean "min()"? Unimportant. ]

I see the convenience here, but doubt I'd ever do that myself. I'd write the above like this:

while bytes_left > 0: buf = f.read(max(bytes_left, page_size)) if buf == 0: break process(buf) # updates bytes_left

I'm kind of picky about doing things exactly as often as required and no more. Especially things that call another facility.

read(0) itself must internally have a check for size == 0 anyway, so it's not like the overall system is less complex. If we're unlucky it could trickle all the way down to an OS system call to read(2) (UNIX, substitute as suitable elsewhere) and for a no-op that would be overkill by far. The only way the read() implementation would avoid that is by doing the test on size anyway. But since read() is opaque IMO it is better to avoid it at the upper level if we know it will produce nothing.

Which leaves me unconvinced of the utility of this mode.

Cheers,

Cameron Simpson <cs at zip.com.au> DoD#743 http://www.cskk.ezoshosting.com/cs/

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