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Title: csv.reader() does not support escaped newline when quoting=csv.QUOTE_NONE
Type: behavior Stage: resolved
Components: Library (Lib) Versions: Python 3.4
Status: closed Resolution: fixed
Dependencies: Superseder:
Assigned To: Nosy List: kalaxy, lukasz.langa, maciej.szulik, mjohnson, python-dev, r.david.murray, serhiy.storchaka, terry.reedy, xflr6
Priority: normal Keywords: patch

Created on 2012-09-12 04:49 by kalaxy, last changed 2022-04-11 14:57 by admin. This issue is now closed.

File name Uploaded Description Edit kalaxy, 2012-09-12 04:49 Script exhibiting bug. maciej.szulik, 2012-10-09 20:41 Script exhibiting bug for py3k.
csv.patch mjohnson, 2013-03-20 01:43 review
Messages (10)
msg170352 - (view) Author: Kalon Mills (kalaxy) Date: 2012-09-12 04:49
cvs.reader improperly prematurely ends row parsing when parsing a row with an escaped newline but with quoting turned off.  cvs.reader properly handles quoted newlines.  cvs.writer properly handles writing escaped unquoted newlines so only the reader has an issue.

Given a dialect with escapechar='\\', quoting=csv.QUOTE_NONE, lineterminator='\n':

writer.writerow(['one\nelement']) will correctly write 'one\\\nelement\n'

however pass that back into a reader and it will produce two rows: ['one\n'] ['element']

I would expect the reader to parse it correctly and return the original value of ['one\nelement']
I've attached a test script that exhibits the improper behavior.  It uses a dialect to set an escapechar and disable quoting.
msg172521 - (view) Author: Maciej Szulik (maciej.szulik) * (Python triager) Date: 2012-10-09 20:41
I've confirmed that bug in the latest repo version, still exists. I attach patch for py3k. 
I'll try to have a look at it in the current version, as soon as it will be fixed I'll port it to 2.7.
msg175900 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2012-11-18 18:33
CSV is not well defined format. What you expect to read from csv.reader(['one', 'two'])? If two rows ['one'] and ['two'], than the reader in its own right and there is no bug which can be fixed.
msg184415 - (view) Author: Kalon Mills (kalaxy) Date: 2013-03-18 02:49
Serhiy,  sorry I'm not sure I understand your question.  But if you take a look at the script that exhibits the problem I think the bug that I'm reporting becomes more clear.

Namely, using the dialect configuration shown in the script, the round trip conversion from string through writer then through the reader back to string is inconsistent.  The reader should return as output the same input that was given to the corresponding writer and this is not the case.  

So even if CVS in not well defined I believe the writer and reader should at least be consistent.
msg184720 - (view) Author: Michael Johnson (mjohnson) * Date: 2013-03-20 01:43
On input, the reader sees a line like 


from the file iterator and successfully escapes the newline character, but still interprets the end of the string as the end of a record.  I've attached a patch that modifies this behavior, so that encountering the end of a string immediately after an escaped \r or \n is does not begin a new record.
msg184723 - (view) Author: Roundup Robot (python-dev) (Python triager) Date: 2013-03-20 02:42
New changeset 940748853712 by R David Murray in branch 'default':
#15927: Fix cvs.reader parsing of escaped \r\n with quoting off.
msg184724 - (view) Author: R. David Murray (r.david.murray) * (Python committer) Date: 2013-03-20 02:44
Although this is clearly a bug fix, it also represents a behavior change that could cause a working program to fail.  I have therefore only applied it to 3.4, but I'm open to arguments that it should be backported.

Thanks for the patch, Michael.
msg303387 - (view) Author: Terry J. Reedy (terry.reedy) * (Python committer) Date: 2017-09-30 03:05
This issue was 'reopened' by #31590.

I can understand inconsistency as a 'design bug', but design bugs are not code bugs, and fixing a design bugs is an enhancement issue, not a behavior issue.

It is not clear to me why, with the specified dialect,  "writer.writerow(['one\nelement'])" is correct in writing 'one\\\nelement\n'.  The doc for Dialect.excapechar says "A one-character string used by the writer to escape the delimiter if quoting is set to QUOTE_NONE and the quotechar if doublequote is False."  Yes, quoting is set to QUOTE_NONE, but \n is the lineterminator, not the delimiter (or the quotechar).  It looks to me that escaping the lineterminator might be a bug.

In any case, 'one\nelement' and 'one\\\nelement' are each 2 physical lines.  I don't see anything in the doc about csv.reader joining physical lines into 'logical' lines the way that compile() does.
msg309811 - (view) Author: Sebastian Bank (xflr6) Date: 2018-01-11 14:57
I am not sure about the design vs. code bug distinction, but what makes me think this should be fixed is primarily the broken round-trip (already mentioned above): 

>>> import io, csv
>>> def roundtrip(value, **fmtparams):
        with io.BytesIO() as f:
             csv.writer(f, **fmtparams).writerow([value])
             return next(csv.reader(f, **fmtparams))
>>> roundtrip('spam\neggs', quoting=csv.QUOTE_NONE, escapechar='\\')

Furthermore, there is the inconsistency between Python 2 and 3, now that this has been fixed in 3.4.

I agree that the documentation of Dialect.escapechar is not in line with the code (in both Python 2 and Python 3): How about changing it to something along the following lines (TODO: reformulate according to how exactly Dialect.lineterminator affects this)?

"to escape the delimiter, \r, \n, and the quotechar if quoting is set to QUOTE_NONE
and the quotechar for all other quoting styles if doublequote is False":

>>> def write_csv(value, **fmtparams):
        with io.BytesIO() as f:
            csv.writer(f, **fmtparams).writerow([value])
            return f.getvalue()
>>> write_csv('spam\reggs', quoting=csv.QUOTE_NONE, escapechar='\\')
>>> write_csv('spam\neggs', quoting=csv.QUOTE_NONE, escapechar='\\')
>>> write_csv('spam"eggs', quoting=csv.QUOTE_NONE, escapechar='\\')
>>> write_csv('spam"eggs', quoting=csv.QUOTE_NONE, quotechar=None, escapechar='\\')
>>> write_csv('spam"eggs', escapechar='\\', doublequote=False)

> In any case, 'one\nelement' and 'one\\\nelement' are each 2 physical lines. 
> I don't see anything in the doc about csv.reader joining physical lines
> into 'logical' lines the way that compile() does.

How about the following?


    The number of lines read from the source iterator. This is not the same as the number of records returned, as records can span multiple lines."

"On reading, the escapechar removes any special meaning from the following character."

>>> write_csv('spam\neggs', quoting=csv.QUOTE_NONE)  # with delimiter, \r, \n, and quotechar
Traceback (most recent call last):
Error: need to escape, but no escapechar set

>>> roundtrip('spam\neggs')

>>> write_csv('spam\neggs')
msg309846 - (view) Author: Sebastian Bank (xflr6) Date: 2018-01-12 10:17
To be complete, the docs of Dialect.escapechar should probably also say that it is used to escape itself.
However, note that csw.writer currently only does this with csv.QUOTE_NONE (breaking round-trip otherwise: #12178).
Date User Action Args
2022-04-11 14:57:35adminsetgithub: 60131
2018-01-12 10:17:18xflr6setmessages: + msg309846
2018-01-11 14:57:58xflr6setnosy: + xflr6
messages: + msg309811
2017-09-30 03:06:00terry.reedysetnosy: + terry.reedy
messages: + msg303387
2013-03-20 02:44:33r.david.murraysetstatus: open -> closed

assignee: lukasz.langa ->
versions: - Python 2.7, Python 3.2, Python 3.3
nosy: + r.david.murray

messages: + msg184724
resolution: fixed
stage: resolved
2013-03-20 02:42:03python-devsetnosy: + python-dev
messages: + msg184723
2013-03-20 01:43:57mjohnsonsetfiles: + csv.patch

nosy: + mjohnson
messages: + msg184720

keywords: + patch
2013-03-18 02:49:09kalaxysetmessages: + msg184415
2012-11-18 18:33:13serhiy.storchakasetmessages: + msg175900
2012-10-09 22:36:47serhiy.storchakasetnosy: + serhiy.storchaka
2012-10-09 20:45:31lukasz.langasetassignee: lukasz.langa

nosy: + lukasz.langa
versions: + Python 3.2, Python 3.3
2012-10-09 20:41:52maciej.szuliksetfiles: +
versions: + Python 3.4
nosy: + maciej.szulik

messages: + msg172521
2012-09-13 03:32:08chris.jerdoneksetcomponents: + Library (Lib), - None
title: cvs.reader does not support escaped newline when quoting=cvs.QUOTE_NONE -> csv.reader() does not support escaped newline when quoting=csv.QUOTE_NONE
2012-09-12 04:49:29kalaxycreate