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

[Thomas Wouters]

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>

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