Joining byte list with python

Question

I'm trying to develop a tool that read a binary file, makes some changes and save it. What I'm trying to do is make a list of each line in the file, work with several lines and then join the list again.

This is what I tried:

file = open('myFile.exe', 'r+b')

aList = []
for line in f:
    aList.append(line)

#Here im going to mutate some lines.

new_file = ''.join(aList)

and give me this error:

TypeError: sequence item 0: expected str instance, bytes found

which makes sense because I'm working with bytes.

Is there a way I can use join function o something similar to join bytes? Thank you.

for what it's worth, you could do aList = f.readlines() or aList = list(f) — mgilson
– mgilson, Commented Jun 12, 2013 at 14:29
Does it make sense to talk about "lines" in a binary file? Usually we add newlines for legibility, and that doesn't apply to binary files. I wonder if you'd be better off working with fixed-length strings, like every 80 bytes. (Just a thought.) — sea-rob
– sea-rob, Commented Jun 12, 2013 at 14:33
@sea-rob PDFs for example are a weird mishmash of text and binary format, where sometimes line breaks matter and you can have bytes that don't form valid UTF-8. — Alien426
– Alien426, Commented Aug 21, 2023 at 6:54

Andrew Clark · Accepted Answer · 2013-06-12 14:38:04Z

150

Perform the join on a byte string using b''.join():

>>> b''.join([b'line 1\n', b'line 2\n'])
b'line 1\nline 2\n'

answered Jun 12, 2013 at 14:38

Andrew Clark

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mawimawi · Accepted Answer · 2013-06-12 14:30:44Z

3

Just work on your "lines" and write them out as soon as you are finished with them.

file = open('myFile.exe', 'r+b')
outfile = open('myOutfile.exe', 'wb')

for line in f:
    #Here you are going to mutate the CURRENT line.
    outfile.write(line)
file.close()
outfile.close()

answered Jun 12, 2013 at 14:30

mawimawi

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1 Comment

user2130898 Over a year ago

I didn't think about this solution, this is much easier than mine. Thanks!

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