How to write text into a file opened in binary mode using Python 3?

Question

I run the following code with Python 3.5.1 on Windows 7.

with open('foo.txt', 'wb') as f:
    print(b'foo\nbar\n', file=f)

I get the following error.

Traceback (most recent call last):
  File "foo.py", line 2, in <module>
    print(b'foo\nbar\n', file=f)
TypeError: a bytes-like object is required, not 'str'

My intention is to write text in a file such that all '\n' appears as LF (as opposed to CRLF) in the file.

What's wrong in my code above? What is the right way to write text into a file opened in binary mode?

Mark Tolonen · Accepted Answer · 2017-09-20 03:34:25Z

2

You don't need binary mode. Specify the newline when opening the file. The default is universal newline mode, which translates newlines to/from the platform default. newline='' or newline='\n' specifies untranslated mode:

with open('foo.txt', 'w', newline='\n') as f:
    print('foo', file=f)
    print('bar', file=f)

with open('bar.txt', 'w', newline='\r') as f:
    print('foo', file=f)
    print('bar', file=f)

with open('foo.txt','rb') as f:
    print(f.read())

with open('bar.txt','rb') as f:
    print(f.read())

Output (on Windows):

b'foo\nbar\n'
b'foo\rbar\r'

edited Sep 20, 2017 at 3:34

answered Sep 11, 2017 at 13:41

Mark Tolonen

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4 Comments

Lone Learner Over a year ago

That is going to CRLF on Windows instead of writing LF. I want LF to be written even on Windows.

Mark Tolonen Over a year ago

@LoneLearner did you try it? Python 3 does it's own line handling and it writes only LF on Windows with this code. I'm on Windows and verified it with a hex dump.

Mark Tolonen Over a year ago

@LoneLearner Added proof.

Milk_Shaykh Over a year ago

@MarkTolonen Thank you so much this really helped me :).

Ignacio Vazquez-Abrams · Accepted Answer · 2016-04-07 04:51:27Z

0

print() does things to the object passed to it. Avoid using it for binary data.

f.write(b'foo\nbar\n')

answered Apr 7, 2016 at 4:51

Ignacio Vazquez-Abrams

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