Writing to io.BytesIO in csv fails in python3

I am trying to write python 2/3 compatible code to write strings to csv file object. This code:

line_as_list = [line.encode() for line in line_as_list]
writer_file =  io.BytesIO()
writer = csv.writer(writer_file, dialect=dialect, delimiter=self.delimiter)
for line in line_as_list:
    assert isinstance(line,bytes)
    writer.writerow(line)

Gives this error on Python3:

>           writer.writerow(line)
E           TypeError: a bytes-like object is required, not 'str'

But assert has no problem with the type, so why is csv creating an error?

Can't I use BytesIO only for both Python 2 and 3? Where is the problem here?

In Python3 csv.writer expects a file-like object opened in text mode. In Python2, csv.writer expects a file-like object opened in binary mode.

Therefore, in Python3, use io.StringIO, while in Python2 use io.BytesIO:

import io
import csv
import sys
PY3 = sys.version_info[0] == 3

line_as_list = [u'foo', u'bar']
encoding = 'utf-8'

if PY3:
    writer_file =  io.StringIO()
else:
    writer_file =  io.BytesIO()
    line_as_list = [line.encode(encoding) for line in line_as_list]

writer = csv.writer(writer_file, dialect='excel', delimiter=',')
writer.writerow(line_as_list)
content = writer_file.getvalue()

if PY3:
    content = content.encode(encoding)

print(type(content))
print(repr(content))

In Python3 the code above prints