[Python-Dev] file.next() vs. file.readline()

[Guido van Rossum]

I'd say go right ahead and submit a change to SF (and then after it's
reviewed you can check it in yourself :-).

The only reason I can think of why this isn't done yet would be that
nobody thought of it.

Of course there are other ill-advised combinations, like write
followed by read for which stdio doesn't guarantee any result in
particular. In Py3K I want to revise the whole I/O stack to be
independent from C stdio (except on those platforms where that's the
only I/O you have.)

--Guido

On 1/4/06, Thomas Wouters <thomas at xs4all.net> wrote:
>
> Twice now, I've been bitten by the 'mixing file-iteration with readline'
> issue. (Yeah, twice.. Good thing I don't write anything important in
> Python ;) I've also seen a number of newbies bitten by the same thing. The
> issue, for those that aren't aware, is that when mixing file-iteration and
> readline (or readlines or such), you run the risk of getting data in the
> wrong order. This is because file-iteration uses an 8k buffer, while
> readline doesn't. It's not really a big deal, once you understand it's not
> wise to mix iteration and the readline(s) methods.
>
> I do wonder, though, why Python doesn't take care to raise an exception when
> readline is called with 'stuff' in the iteration-buffer. A cursory look at
> the source doesn't reveal any glaring problems. It's a single check,
> possibly two, with good locality of reference. Also, raising an exception
> when there is stuff in the buffer would only make mixing iteration/readline
> an error when you would actually, in fact, lose or mess up data. In other
> words, it would only raise exceptions for existing cases that are already
> broken.
>
> Is there something I've missed that makes the check undesireable or
> unfeasible? Or should I just assume no on has gotten around to it (or could
> be bothered), and just submit a patch? :)
>
> (Making readline and friends use the same buffer may seem the better
> solution to some, but I'm sure there's a whole discussion behind that, about
> whether to buffer or not. All non-iteration routines in fileobject.c take
> pretty good care not to read too much, and I choose to see that as
> explicitly designed that way.)
>
> Absent-ly y'rs,
> --
> Thomas Wouters <thomas at xs4all.net>
>
> Hi! I'm a .signature virus! copy me into your .signature file to help me spread!
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--
--Guido van Rossum (home page: http://www.python.org/~guido/)