In article <[EMAIL PROTECTED]>, Gustavo Córdova Avila <[EMAIL PROTECTED]> wrote:
David Bolen wrote:
Jp Calderone <[EMAIL PROTECTED]> writes:
def nonBlockingReadAll(fileObj): bytes = [] while True: b = fileObj.read(1024) bytes.append(b) if len(b) < 1024: break return ''.join(bytes)
Wouldn't this still block if the input just happened to end at a multiple of the read size (1024)?
-- David
No, it'll read up to 1024 bytes or as much as it can, and then return an apropriatly sized string.
Depends. I don't believe the original post mentioned that the file is a pipe, socket or similar, but it's
It did actually specifically mention files.
Erm, you mean they operate at infinite speed? Wrong. There's a definite risk that a process will block when reading from disk, and the reason that (Unix) files can be the object of select() calls is to allow users to perform non-blocking reads and continue to process while waiting for disk data to arrive.kind of implied by the use of select() also mentioned. It's also kind of implied by use of the term "block" - disk files don't block.
I agree the blocking period is *more predictable* than for network data, but "disk file access won't cause a process to block" is a misleading and incorrect, though frequent, assumption.
I agree that it's difficult to actually write asynchronous disk I/O in such a way as to improve performance, but it can be done. Except on platforms where select() can't be applied to disk files, of course. In practice most applications that need the scalability will use threading or multiple processes.
If we are indeed talking about a pipe or something that really can block, and you call fileobject.read(1024), it will block until it gets 1024 bytes.
Donn Cave, [EMAIL PROTECTED]
Disk access can block. When access is made to a file object in non-blocking mode it will return whatever data there are immediately available in the buffers, up to the number of bytes requested. If the buffers are currently empty the process will generate an error (which in Python becomes an exception).
regards Steve -- http://www.holdenweb.com http://pydish.holdenweb.com Holden Web LLC +1 800 494 3119 -- http://mail.python.org/mailman/listinfo/python-list
