[Python-Dev] Fuzziness in io module specs (original) (raw)

[Python-Dev] Fuzziness in io module specs - PEP update proposition

Pascal Chambon chambon.pascal at gmail.com
Sun Sep 20 11:48:49 CEST 2009

Hello

After weighing up here and that, here is what I have come with. Comments and issue notifications more than welcome, of course. The exception thingy is not yet addressed.

Regards, Pascal

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:

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

(same for BufferedIOBase.truncate(pos: 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 n, the extra data is discarded. If the file was previously shorter than n, 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).

Proposition 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: bytes) -> int

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

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