line terminator issue (again) on windows when writing to_csv()

[briochh]

Another addition to windows line-end saga:
Changes in Pandas 0.24.0, related to #20353, seem to cause some strange behaviour in this not so uncommom edge case:

Problem description

When using df.to_csv() to write to an exisitng file handle without explicitly setting newline='' (or newline='\n') in the open statement an extra '\r' is added to the default (on windows) '\r\n' line-end:

data = pd.DataFrame({"string_with_lf": ["a\nbc"], "string_with_crlf": ["a\r\nbc"]})
with open("test2.csv", mode='w') as f:
    data.to_csv(f, index=False)
with open("test2.csv", mode='rb') as f:
    print(f.read())

Expected output

b'string_with_lf,string_with_crlf\r\n"a\nbc","a\r\nbc"\r\n'

Current output

b'string_with_lf,string_with_crlf\r\r\n"a\r\nbc","a\r\r\nbc"\r\r\n'

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None
python: 3.6.7.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 79 Stepping 1, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None
pandas: 0.24.0
pytest: 4.0.2
pip: 18.1
setuptools: 40.6.3
Cython: 0.29.2
numpy: 1.15.4
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 7.2.0
sphinx: 1.8.2
patsy: 0.5.1
dateutil: 2.7.5
pytz: 2018.7
blosc: None
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.8
feather: None
matplotlib: 3.0.2
openpyxl: 2.5.12
xlrd: 1.2.0
xlwt: 1.3.0
xlsxwriter: 1.1.2
lxml.etree: 4.2.5
bs4: 4.6.3
html5lib: 1.0.1
sqlalchemy: 1.2.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None

[gfyoung]

cc @deflatSOCO @jreback

So the reason for this behavior is two-fold:

1. We explicitly set the terminator (in this case, \r\n) when writing to the file because this is specified when you called to_csv without a specific line_terminator.
2. Because you are using f as a file handler with newline=None, Python then proceeds to replace each \n with \r\n. Thus, what you get then is \r\r\n.

Admittedly, this is an unfortunate corner case because we have no information going in what the newline terminator should be i.e., how would we know that we should use \n instead of \r\n for the line terminator (so that we avoid \r\r\n).

I'm inclined to suggest that we document this behavior in more detail for Windows because the workaround for your case is clear. Not sure if there is a straightforward fix to make all parties happy here. Thoughts?

[jorisvandenbossche]

cc @chris-b1

[chris-b1]

Is there some way to introspect a file object for its newlines mode? Not finding an obvious way. I do think we need to fix this

[gfyoung]

I do think we need to fix this

@chris-b1 : I don't think anyone is disagreeing with you on that. However, having read the documentation for open, introspection does not look easy, if possible, and if it isn't, we find ourselves caught between a rock and a hard place regarding this issue and #20353.

At the very least, something should be documented to address this caveat for Windows if we can't find a way to introspect on a file object.

[gfyoung]

Given this was a behavior change in 0.24.0, while I don't see an easy fix (if any) for this, we should at least document this caveat for Windows in 0.24.2.

[CaselIT]

This may be related to #25311

[TomAugspurger]

Are people on-board with documentation this as the expected behavior, and giving guidance for how to avoid it? If so, does anyone have time to put up a PR in time for 0.24.2?

[chris-b1]

If we're shooting for next for 0.24.2 , I'll have time this weekend to look.

[chris-b1]

It does seem like there isn't a way to work around this. python's documentation puts it on the user to pass newline='' if using a file object with the csv writer, I'll have a PR to do the same here
https://docs.python.org/3/library/csv.html#id3