read_csv encode operation on source

[amcoder-mb]

Code Sample, a copy-pastable example if possible

>>> import pandas as pd
>>> df = pd.read_csv("historical_dataset.csv")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3/dist-packages/pandas/io/parsers.py", line 709, in parser_f
    return _read(filepath_or_buffer, kwds)
  File "/usr/lib/python3/dist-packages/pandas/io/parsers.py", line 449, in _read
    parser = TextFileReader(filepath_or_buffer, **kwds)
  File "/usr/lib/python3/dist-packages/pandas/io/parsers.py", line 818, in __init__
    self._make_engine(self.engine)
  File "/usr/lib/python3/dist-packages/pandas/io/parsers.py", line 1049, in _make_engine
    self._engine = CParserWrapper(self.f, **self.options)
  File "/usr/lib/python3/dist-packages/pandas/io/parsers.py", line 1695, in __init__
    self._reader = parsers.TextReader(src, **kwds)
  File "pandas/_libs/parsers.pyx", line 402, in pandas._libs.parsers.TextReader.__cinit__
  File "pandas/_libs/parsers.pyx", line 718, in pandas._libs.parsers.TextReader._setup_parser_source
FileNotFoundError: File b'historical_dataset.csv' does not exist

Problem description

Noticed that when source does not exist, there is a leading 'b' on the source on the error message. For Python3, is it necessary to do the encode operation on line 667 of https://github.com/pandas-dev/pandas/blob/master/pandas/_libs/parsers.pyx? Works fine for Python2.

Output of pd.show_versions()

``` INSTALLED VERSIONS ------------------ commit : None python : 3.7.4.final.0 python-bits : 64 OS : Linux OS-release : 5.0.0-31-generic machine : x86_64 processor : x86_64 byteorder : little LC_ALL : en_US.UTF-8 LANG : en_US.UTF-8 LOCALE : en_US.UTF-8

pandas : 0.25.1
numpy : 1.14.5
pytz : 2018.5
dateutil : 2.8.0
pip : 19.2.3
setuptools : 41.2.0
Cython : 0.29.13
pytest : None
hypothesis : None
sphinx : 1.7.6
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.2.3
html5lib : None
pymysql : 0.9.3
psycopg2 : None
jinja2 : 2.10
IPython : None
pandas_datareader: None
bs4 : None
bottleneck : None
fastparquet : None
gcsfs : None
lxml.etree : 4.2.3
matplotlib : 2.2.2
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pytables : None
s3fs : None
scipy : 1.1.0
sqlalchemy : 1.3.8
tables : None
xarray : None
xlrd : None
xlwt : None
xlsxwriter : None

</details>

[gfyoung]

@abellerarj : The reason for the b is that we encode the name of the file to account for special characters cross-OS (xref: #24758). In Python2, the visual representation of encoded and non-encoded strings looked one and the same, whereas in Python3, a distinction is made visually.

[gfyoung]

Now for the error message, we don't necessarily have to pass in the encoded version. We could use the unencoded one instead. You are welcome to investigate.

[jbrockmendel]

@gfyoung to get the non-prefixed filename, would we need to reverse the encoding done in #24758? that is now done in a .c file that i dont want to futz with

[gfyoung]

would we need to reverse the encoding done in #24758

That would a be logical first step to investigate.

[gfyoung]

Fat fingers, my bad.