csv.writer converts None to '""\n' when it is first line, otherwise '\n'

[Licht-T]

BPO	32255
Nosy	@bitdancer, @serhiy-storchaka, @nitishch, @Licht-T
PRs	bpo-32255: Fix inconsistent behavior when csv.writer writes None #4769 [3.6] bpo-32255: Always quote a single empty field when write into a CSV file. (GH-4769) #4810

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2017-12-12.10:56:58.260>
created_at = <Date 2017-12-08.14:43:54.282>
labels = ['3.7', 'type-bug', 'library']
title = 'csv.writer converts None to \'""\\n\' when it is first line, otherwise \'\\n\''
updated_at = <Date 2017-12-12.10:56:58.259>
user = 'https://github.com/Licht-T'

bugs.python.org fields:

activity = <Date 2017-12-12.10:56:58.259>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2017-12-12.10:56:58.260>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2017-12-08.14:43:54.282>
creator = 'licht-t'
dependencies = []
files = []
hgrepos = []
issue_num = 32255
keywords = ['patch']
message_count = 12.0
messages = ['307851', '307939', '307940', '307941', '307984', '307986', '307997', '308009', '308050', '308102', '308103', '308109']
nosy_count = 4.0
nosy_names = ['r.david.murray', 'serhiy.storchaka', 'nitishch', 'licht-t']
pr_nums = ['4769', '4810']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32255'
versions = ['Python 3.6', 'Python 3.7']

[Licht-T]

Inconsistent behavior while reading a single column CSV.
I have the patch and waiting for the CLA response.

# Case 1
## Input

import csv
fp = open('test.csv', 'w')
w = csv.writer(fp)
w.writerow([''])
w.writerow(['1'])
fp.close()

## Output

""
1

# Case 2
## Input

import csv
fp = open('test.csv', 'w')
w = csv.writer(fp)
w.writerow(['1'])
w.writerow([''])
fp.close()

## Output

1

[nitishch]

Which scenario you think is the wrong behaviour in this case? First one or second one?

I don't know much about csv module, but I thought it was a deliberate choice made to quote all empty lines and hence considered the second scenario as buggy. But your pull requests seems to fix the first case. Am I missing something here?

[Licht-T]

I think the first one is buggy and there are two reasons.

1. The both are valid CSV. The double quoting is unnecessary. Some other applications, eg. Excel, does not use the double quoting.
   Also, the current implementation make to quote only if the string is '' and the output is at the first line.

2. '' is not quoted when the two columns case.
   ## Input:

import csv
fp = open('test.csv', 'w')
w = csv.writer(fp, dialect=None)
w.writerow(['', ''])
w.writerow(['3', 'a'])
fp.close()

## Output:

,
3,a

These seem inconsistent and the quoting is unnecessary in this case.

# References
http://www.ietf.org/rfc/rfc4180.txt

[Licht-T]

The current implementation does not quote in most case. IOW, the patch which makes all '' is quoted is the breaking change (Note that there are some applications does not use quoting).

[bitdancer]

The second case is indeed the bug, as can be seen by running the examples against python2.7. It looks like this was probably broken by 7901b48 from bpo-23171.

[bitdancer]

Serhiy, since it was your patch that probably introduced this bug, can you take a look? Obviously it isn't a very high priority bug, since no one has reported a problem (even this issue isn't reporting the change in behavior as a *problem* :)

[serhiy-storchaka]

For restoring the 3.4 behavior the single empty field must be quoted. This allows to distinguish a 1-element row with the single empty field from an empty row.

[Licht-T]

Thanks for your investigation!
Would you mind if I create a new patch?