[Python-Dev] TextIOWrapper.tell() (original) (raw)
Antoine Pitrou solipsis at pitrou.net
Wed Jun 30 19:20:34 CEST 2010
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On Wed, 30 Jun 2010 10:03:49 -0700 Guido van Rossum <guido at python.org> wrote:
> Also, please note that values used by seek() and tell() on > text I/O are "opaque cookies". While they can happen to match the > raw binary file position, it is a mere coincidence (or an > implementation detail, at your will). Therefore, reusing tell() values > of a binary file to seek() a TextIOWrapper accessing the same file > is wrong. Well, um, I actually designed it carefully so that bytes offsets would work as text offsets in those cases where they make sense at all.
Ah, this is embarrassing. I always assumed it was an implementation detail since neither the PEP nor the module docs say otherwise.
PEP 3116 clearly says:
“Unlike with raw I/O, the units for .seek() are not specified - some implementations (e.g. StringIO) use characters and others (e.g. TextIOWrapper) use bytes.”
And also:
“.seek(pos: object, whence: int = 0) -> int
Seek to position pos. If pos is non-zero, it must be a cookie
returned from .tell() and whence must be zero.”“it must be a cookie returned from .tell()” here seems to imply that non-zero values of other origin should not be used.
Regards
Antoine.
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