[Python-Dev] Fuzziness in io module specs - PEP update proposition V2 (original) (raw)

Pascal Chambon chambon.pascal at gmail.com
Sun Sep 27 09:58:51 CEST 2009

Hello

Below is a corrected version of the PEP update, adding the start/end indexes proposition and fixing functions signatures. Does anyone disagree with these specifications ? Or can we consider it as a target for the next versions of the io module ? I would have no problem to implement this behaviour in my own pure python FileIO system, however if someone is willing to patch the _fileio implementation, it'd save a lot of time - I most probably won't have the means to setup a C compilation environment under windows and linux, and properly update/test this, before January (when I get freelance...).

I launch another thread on other to-be-discussed IO points B-)

Regards, Pascal

================ PEP UPDATE for new I/O system - v2 ===========

Truncate and file pointer semantics

Rationale :

The current implementation of truncate() always move the file pointer to the new end of file.

This behaviour is interesting for compatibility, if the file has been reduced and the file pointer is now past its end, since some platforms might require 0 <= filepointer <= filesize.

However, there are several arguments against this semantic:

* Most common standards (posix, win32…) allow the file pointer to be
  past the end of file, and define the behaviour of other stream
  methods in this case
* In many cases, moving the filepointer when truncating has no
  reasons to happen (if we’re extending the file, or reducing it
  without going beneath the file pointer)
* Making 0 <= filepointer <= filesize a global rule of the python IO
  module doesn’t seems possible, since it would require
  modifications of the semantic of other methods (eg. seek() should
  raise exceptions or silently disobey when asked to move the
  filepointer past the end of file), and lead to incoherent
  situations when concurrently accessing files without locking (what
  if another process truncates to 0 bytes the file you’re writing ?)

So here is the proposed semantic, which matches established conventions:

IOBase.truncate(n: int = None) -> int

Resizes the file to the size specified by the positive integer n, or by the current filepointer position if n is None.

The file must be opened with write permissions.

If the file was previously larger than size, the extra data is discarded. If the file was previously shorter than size, its size is increased, and the extended area appears as if it were zero-filled.

In any case, the file pointer is left unchanged, and may point beyond the end of file.

Note: trying to read past the end of file returns an empty string, and trying to write past the end of file extends it by zero-ing the gap. On rare platforms which don't support file pointers to be beyond the end of file, all these behaviours shall be faked thanks to internal storage of the "wanted" file pointer position (silently extending the file, if necessary, when a write operation occurs).

Propositions of doc update

RawIOBase.read(n: int) -> bytes

Read up to n bytes from the object and return them. Fewer than n bytes may be returned if the operating system call returns fewer than n bytes. If 0 bytes are returned, and n was not 0, this indicates end of file. If the object is in non-blocking mode and no bytes are available, the call returns None.

RawIOBase.readinto(b: bytearray, [start: int = None], [end: int = None]) -> int

start and end are used as slice indexes, so that the bytearray taken into account is actually range = b[start:end] (or b[start:], b[:end] or b[:], depending on the arguments which are not None).

Read up to len(range) bytes from the object and store them in b, returning the number of bytes read. Like .read, fewer than len(range) bytes may be read, and 0 indicates end of file if len(range) is not 0. None is returned if a non-blocking object has no bytes available. The length of b is never changed.

-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20090927/49657bb8/attachment-0001.htm>

More information about the Python-Dev mailing list