File mode in `to_csv` is ignored, when passing a file object instead of a path

Code Sample, a copy-pastable example if possible

>>> import pandas as pd
>>> df = pd.read_csv("example.csv")
>>> df.head()
   just  a  file
0     1  2     3
1     4  5     6
2     7  8     9
>>> with open("someother.csv", "wb") as f:
...     df.to_csv(f, mode="wb")
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/usr/lib/python3.6/site-packages/pandas/core/frame.py", line 1524, in to_csv
    formatter.save()
  File "/usr/lib/python3.6/site-packages/pandas/io/formats/format.py", line 1652, in save
    self._save()
  File "/usr/lib/python3.6/site-packages/pandas/io/formats/format.py", line 1740, in _save
    self._save_header()
  File "/usr/lib/python3.6/site-packages/pandas/io/formats/format.py", line 1708, in _save_header
    writer.writerow(encoded_labels)
TypeError: a bytes-like object is required, not 'str'

Problem description

When passing a file opened in binary mode to df.to_csv and also passing mode='wb', this mode is ignored. I think it’s because of these lines: https://github.com/pandas-dev/pandas/blob/master/pandas/io/common.py#L407-L411 and these ones: https://github.com/pandas-dev/pandas/blob/master/pandas/io/formats/format.py#L1660-L1662

It seems that is_text isn’t passed, and so it assumes the default value of True

Expected Output

A file should just be written.

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None python: 3.6.4.final.0 python-bits: 64 OS: Linux OS-release: 4.14.20-1-lts machine: x86_64 processor: byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8

pandas: 0.22.0 pytest: 3.4.0 pip: 9.0.1 setuptools: 38.5.1 Cython: 0.27.3 numpy: 1.14.0 scipy: 1.0.0 pyarrow: None xarray: None IPython: 6.2.1 sphinx: 1.7.0 patsy: 0.4.1 dateutil: 2.6.1 pytz: 2018.3 blosc: None bottleneck: None tables: None numexpr: None feather: None matplotlib: 2.1.2 openpyxl: None xlrd: None xlwt: None xlsxwriter: None lxml: None bs4: 4.6.0 html5lib: 1.0b10 sqlalchemy: 1.2.3 pymysql: None psycopg2: None jinja2: 2.10 s3fs: 0.1.2 fastparquet: 0.1.3 pandas_gbq: None pandas_datareader: None

[Huite]

I’ve downgraded to 0.23.0 to check, and 0.23.0 works as expected. It appears to be specific to 0.23.1.

[remram44]

I’m very confused about this, because passing a io.BytesIO to pandas will not work (so whatever detection you do isn’t very good) but passing a simple file-like object with a write() method will make to_csv() write bytes to it for some reason. Why to_csv() puts bytes into anything is beyond me.