Bugs item #645594, was opened at 2002-11-29 06:13
Message generated for change (Comment added) made by facundobatista
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Category: Documentation
Group: Python 2.2.2
>Status: Closed
>Resolution: Wont Fix
Priority: 5
Submitted By: Dmitry Vasiliev (hdima)
Assigned to: Nobody/Anonymous (nobody)
Summary: for lin in file: file.tell() tells wrong
Initial Comment:
Consider following piece of code:
f = file("test.txt", "rb")
pos = f.tell()
for line in f:
print "%u: '%s'" % (pos, line)
pos = f.tell()
During the code execution we have following result:
0 'Line 1'
63 'Line 2'
63 'Line 3'
...
63 'Line 9'
However, following piece of code works fine:
f = file("test.txt", "rb")
while True:
pos = f.tell()
line = f.readline()
if line == "":
break
print "%u: '%s'" % (pos, line)
It prints:
0 'Line 1'
7 'Line 2'
14 'Line 3'
...
56 'Line 9'
It seems a file iterator makes file.tell() to tell
positions of some internal blocks used by the iterator.
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>Comment By: Facundo Batista (facundobatista)
Date: 2005-01-11 00:15
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Closing it, check bug 1036626 for rationale.
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Comment By: Johannes Gijsbers (jlgijsbers)
Date: 2004-11-07 16:29
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Closed accoding to rationale in #1036626.
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Comment By: Brett Cannon (bcannon)
Date: 2003-05-22 01:09
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As suggested by Just and Dimtry I am reclassifying this as a doc bug.
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Comment By: Dmitry Vasiliev (hdima)
Date: 2003-01-08 07:17
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I agree. Unfortunately a small patch doesn't work here. :-(
But I suggest to recategorize it as a documentation bug.
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Comment By: Just van Rossum (jvr)
Date: 2003-01-05 18:25
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This bug is very similar to #524804, which was closed as
"won't fix". Unless it's recategorized as a documentation
bug, I suggest to close this one as a duplicate.
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Comment By: Dmitry Vasiliev (hdima)
Date: 2002-12-04 08:24
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File.next() uses readahead buffer (8192 bytes for now).
Calling file.seek() drops the buffer, but other operations
don't. I think it's possible for file.tell() to return right
result (I'll try to make a patch). But all these quirks
should be documented.
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Comment By: Jack Jansen (jackjansen)
Date: 2002-11-29 19:07
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Actually, all file operations behave the same (well, all: I tried one:
readline():-): they behave as if the whole file has been read. I.e.
file.readline() within a "for line in file" will return an empty
string.
If a file iterator behaves as if it has read the complete file at once on
instantiation (never mind what it actually does: if it *behaves* as if this
happens) I think this should just be documented and that's it.
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Comment By: Tim Peters (tim_one)
Date: 2002-11-29 13:57
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Possibly. Maybe something fancier could be done too, to
give "expected" results. although I don't know how to
approach that on Windows for text-mode files (byte
arithmetic on seek/tell results doesn't work at all for
those). I'd take it to Python-Dev.
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Comment By: Martin v. Löwis (loewis)
Date: 2002-11-29 13:01
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Shouldn't tell raise an exception then?
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Comment By: Tim Peters (tim_one)
Date: 2002-11-29 12:42
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"for x in file" does do its own chunking under the covers, for
speed, and seek() and tell() are indeed not to be used on a
file being iterated over in this way.
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Comment By: Dmitry Vasiliev (hdima)
Date: 2002-11-29 11:46
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I'll try to dig in.
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Comment By: Martin v. Löwis (loewis)
Date: 2002-11-29 11:30
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Would you like to investigate this further? There is no need
to, but if you find a solution and a patch, there is a better
chance that this is fixed before 2.3.
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