Writing to io.BytesIO in csv fails in python3

Question

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?

Answer 1

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)) 

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In Python3 the code above prints

<class 'bytes'> b'foo,bar\r\n' 

In Python2 the code above prints

<type 'str'> 'foo,bar\r\n'