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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: Relative seeks on string IO
Date: Tue, 06 Sep 2011 16:49:19 -0400
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On 9/6/2011 3:18 AM, Pierre Quentel wrote:

> I am wondering why relative seeks fail on string IO in Python 3.2

Good question.

>      from io import StringIO
>      txt = StringIO('Favourite Worst Nightmare')
>      txt.seek(8) # no problem with absolute seek

Please post code without non-code indents, like so:

from io import StringIO
txt = StringIO('Favourite Worst Nightmare')
txt.seek(8,0) # no problem with absolute seek
txt.seek(0,1) #  0 characters from current position ok, and useless
txt.seek(-2,2) # end-relative gives error message for cur-relative

so someone can copy and paste without deleting indents.
I verified with 3.2.2 on Win7. I am curious what 2.7 and 3.1 do.

What system are you using? Does it have a narrow or wide unicode build? 
(IE, what is the value of sys.maxunicode?)

>      txt.seek(2,1) # 2 characters from current position
>
> raises "IOError: Can't do nonzero cur-relative seeks" (tested with
> Python3.2.2 on WindowsXP)
>
> A seek relative to the end of the string IO raises the same IOError

> Is there any reason why relative seeks on string IO are not allowed in
> Python3.2, or is it a bug that could be fixed in a next version ?

Since StringIO seeks by fixed-size code units (depending on the build), 
making seeking from the current position and end trivial, I consider 
this a behavior bug. At minimum, it is a doc bug. I opened
http://bugs.python.org/issue12922

As noted there, I suspect the limitation in inherited from TextIOBase. 
But I challenge that it should be.

I was somewhat surprised that seeking (from the start) is not limited to 
the existing text. Seeking past the end fills in with nulls. (They are 
typically a nuisance though.)

from io import StringIO
txt = StringIO('0123456789')
txt.seek(15,0) # no problem with absolute seek
txt.write('xxx')
s  = txt.getvalue()
print(ord(s[12]))
# 0
-- 
Terry Jan Reedy