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

[Pascal Chambon]

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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