Miles Kaufmann wrote:
On Jul 28, 2009, at 6:30 AM, Michele Petrazzo wrote:
Hi list,
I'm trying to port a my library to python 3, but I have a problem with a
the stringio.seek:
the method not accept anymore a value like pos=-6 mode=1, but the "old"
(2.X) version yes...
The error:
File "/home/devel/Py3/lib/python3.0/io.py", line 2031, in seek
return self._seek(pos, whence)
IOError: Can't do nonzero cur-relative seeks
How solve this?
In Python 2, StringIO is a stream of bytes (non-Unicode characters). In
Python 3, StringIO is a stream of text (Unicode characters). In the
early development of Python 3 (and 3.1's _pyio), it was implemented as a
TextIOWrapper over a BytesIO buffer. TextIOWrapper does not support
relative seeks because it is difficult to map the concept of a "current
position" between bytes and the text that it encodes, especially with
variable-width encodings and other considerations. Furthermore, the
value returned from TextIOWrapper.tell isn't just a file position but a
"cookie" that contains other data necessary to restore the decoding
mechanism to the same state. However, for the default encoding (utf-8),
the current position is equivalent to that of the underlying bytes buffer.
In Python 3, StringIO is implemented using an internal buffer of Unicode
characters. There is no technical reason why it can't support relative
seeks; I assume it does not for compatibility with the original Python
TextIOWrapper implementation (which is present in 3.1's _pyio, but not
in 3.0).
Note that because of the different implementations, StringIO.tell() (and
seek) behaves differently for the C and Python implementations:
$ python3.1
>>> import io, _pyio
>>> s = io.StringIO('\u263A'); s.read(1), s.tell()
('☺', 1)
>>> s = _pyio.StringIO('\u263A'); s.read(1), s.tell()
('☺', 3)
It seems to me that this discrepancy might be worth a tracker item.
I wonder why the second implementation is even there if not used.
Two different commiters?
The end result seems to be that, for text streams (including StreamIO),
you *should* treat the value returned by tell() as an opaque magic
cookie, and *only* pass values to seek() that you have obtained from a
previous tell() call. However, in practice, it appears that you *may*
seek StringIO objects relatively by characters using s.seek(s.tell() +
n), so long as you do not use the _pyio.StringIO implementation.
A tracker item could include a request that relative seek be restored if
possible.
tjr
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