BUG: read_csv() wrong reading if comment-character included in na_value

[Waljakov]

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

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

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

---

Code Sample, a copy-pastable example

import pandas as pd
from io import StringIO

dataframe_file = (
    "# this is a comment\n"
    "1,2,3,4\n"
    "1,2,3,4#inline comment\n"
    "1,2#,3,4\n"
    "1,2,#N/A,4\n"
)

dataframe = pd.read_csv(StringIO(dataframe_file), comment="#", na_values="#N/A")

print(dataframe)

Output:

   1  2    3    4
0  1  2  3.0  4.0
1  1  2  NaN  NaN
2  1  2  NaN  NaN

Problem description

On the last row, it should read the 3rd value (#N/A) as NA-Value (NaN) and not as a comment.
But because the value #N/A starts with the comment-character # the rest of the line is interpreted as comment and the last value is not read.

Expected Output

   1  2    3    4
0  1  2  3.0  4.0
1  1  2  NaN  NaN
2  1  2  NaN  4.0

Output of pd.show_versions()

INSTALLED VERSIONS

commit : None
python : 3.7.7.final.0
python-bits : 64
OS : Linux
OS-release : 5.6.8-arch1-1
machine : x86_64
processor :
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.0.3
numpy : 1.18.1
pytz : 2020.1
dateutil : 2.8.1
pip : 20.0.2
setuptools : 46.1.3.post20200330
Cython : 0.29.17
pytest : 5.4.1
hypothesis : 5.8.3
sphinx : 3.0.3
blosc : None
feather : None
xlsxwriter : 1.2.8
lxml.etree : 4.5.0
html5lib : 1.0.1
pymysql : None
psycopg2 : None
jinja2 : 2.11.2
IPython : 7.13.0
pandas_datareader: None
bs4 : 4.9.0
bottleneck : 1.3.2
fastparquet : None
gcsfs : None
lxml.etree : 4.5.0
matplotlib : 3.1.3
numexpr : 2.7.1
odfpy : None
openpyxl : 3.0.3
pandas_gbq : None
pyarrow : None
pytables : None
pytest : 5.4.1
pyxlsb : None
s3fs : None
scipy : 1.4.1
sqlalchemy : 1.3.16
tables : 3.6.1
tabulate : None
xarray : None
xlrd : 1.2.0
xlwt : 1.3.0
xlsxwriter : 1.2.8
numba : 0.49.0

[a9ush]

Add pr template

[Waljakov]

Hey,
sorry, I am a bit confused. Why do you post "add pr template"?
There is a pr template from pandas available here.
Also I don't understand what does this have to do with my issue? Is there something missing that I should add?