Python 2 CSV writer produces wrong line terminator on Windows

According to the its documentation csv.writer should use ‘rn’ as lineterminator by default.

import csv

with open("test.csv", "w") as f:
    writer = csv.writer(f)

    rows = [(0,1,2,3,4),
           (-0,-1,-2,-3,-4),
           ("a","b","c","d","e"),
           ("A","B","C","D","E")]           

    print writer.dialect.lineterminator.replace("r", "\r").replace("n", "\n")
    writer.writerows(rows)
    print writer.dialect.lineterminator.replace("r", "\r").replace("n", "\n")

This prints

rn
rn

as expected. But, the created csv-file uses the lineterminator ‘rrn’

0,1,2,3,4

0,-1,-2,-3,-4

a,b,c,d,e

A,B,C,D,E

Is this a bug or is there something wrong in my usage of csv.writer?

Python version:

ActivePython 2.6.2.2 (ActiveState
Software Inc.) based on Python 2.6.2
(r262:71600, Apr 21 2009, 15:05:37)
[MSC v.1500 32 bit (Intel)] on win32

on Windows Vista

Answers:

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Method 1

In Python 2.x, always open your file in binary mode, as documented. csv writes rn as you expected, but then the underlying Windows text file mechanism cuts in and changes that n to rn … total effect: rrn

From the csv.writer documentation:

If csvfile is a file object, it must be opened with the 'b' flag on platforms where that makes a difference.

There seems to be some reticence about actually uttering the name of the main culprit 🙂

Edit: As mentioned by @jebob in the comments to this answer and based on @Dave Burton’s answer, to handle this case in both Python 2 and 3, you should do the following:

if sys.version_info >= (3,0,0):
    f = open(filename, 'w', newline='')
else:
    f = open(filename, 'wb')

Method 2

Unfortunately, it’s a bit different with the csv module for Python 3, but this code will work on both Python 2 and Python 3:

if sys.version_info >= (3,0,0):
    f = open(filename, 'w', newline='')
else:
    f = open(filename, 'wb')

Method 3

To change the line terminator in Python 2.7 csv writer use

writer = csv.writer(f, delimiter = '|', lineterminator='n')

This is a much simpler way to change the default delimiter from rn.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0