BUG: read_csv() incorrectly parses data in case '\n' is used as a separator

[romanov-o]

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the master branch of pandas.

Reproducible Example

import pandas as pd
import io

df = pd.read_csv(io.StringIO('A\nB\nC'), sep='\n', header=None, names=range(3))

Issue Description

pd.read_csv() incorrectly parses data in case '\n' is used as a separator.

df = pd.read_csv(io.StringIO('A\nB\nC'), sep='\n', header=None, names=range(3))

produces:

   0   1   2
0  A NaN NaN
1  B NaN NaN
2  C NaN NaN

Expected Behavior

Expected:

   0   1   2
0  A   B   C

Installed Versions

INSTALLED VERSIONS

commit : 5f648bf
python : 3.8.10.final.0
python-bits : 64
OS : Darwin
OS-release : 20.6.0
Version : Darwin Kernel Version 20.6.0: Wed Jun 23 00:26:31 PDT 2021; root:xnu-7195.141.2~5/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : None
LOCALE : None.UTF-8

pandas : 1.3.2
numpy : 1.19.2
pytz : 2021.1
dateutil : 2.8.2
pip : 21.2.4
setuptools : 58.0.4
Cython : 0.29.24
pytest : 6.2.5
hypothesis : None
sphinx : 4.1.2
blosc : None
feather : None
xlsxwriter : 3.0.1
lxml.etree : 4.6.3
html5lib : 1.1
pymysql : None
psycopg2 : 2.9.1 (dt dec pq3 ext lo64)
jinja2 : 3.0.1
IPython : 7.27.0
pandas_datareader: None
bs4 : 4.10.0
bottleneck : 1.3.2
fsspec : 2021.08.1
fastparquet : None
gcsfs : None
matplotlib : 3.4.3
numexpr : 2.7.3
odfpy : None
openpyxl : 3.0.8
pandas_gbq : None
pyarrow : None
pyxlsb : None
s3fs : None
scipy : 1.6.2
sqlalchemy : 1.4.23
tables : 3.6.1
tabulate : None
xarray : None
xlrd : 2.0.1
xlwt : 1.3.0
numba : 0.54.0

[leonarduschen]

How would it detect newlines though?

[phofl]

Yeah agreed. I think we should raise if \n is provided as separator.

[leonarduschen]

I'll go ahead and make a PR for that, should be a quick one

[leonarduschen]

Wait - I realized we have the parameter lineterminator - which is \n by default.

import pandas as pd
import io

df = pd.read_csv(io.StringIO('A\nB\nC'), sep='\n', header=None, names=range(3), lineterminator='\r')

gives

   0  1  2
0  A  B  C

Maybe we raise when sep is the same as lineterminator ?

[phofl]

A separator with more than one character forces the python engine, which does not accept a lineterminator