read and write binary files in python

Question

I have a binary file called "input.bin". I am practicing how to work with such files (read them, change the content and write into a new binary file). the contents of input file:

03 fa 55 12 20 66 67 50 e8 ab

which is in hexadecimal notation.

I want to make a output file which is simply the input file with the value of each byte incremented by one.

here is the expected output:

04 fb 56 13 21 67 68 51 e9 ac

which also will be in hexadecimal notation. I am trying to do that in python3 using the following command:

with open("input.bin", "rb") as binary_file:
    data = binary_file.read()
    for item in data:
        item2 = item+1
    with open("output.bin", "wb") as binary_file2:
        binary_file2.write(item2)

but it does not return what I want. do you know how to fix it?

With current indentations you execute write only once - after for loop - so it can write only one value. Open both files in one with at start, and put write inside for loop. — furas
– furas, Commented May 2, 2019 at 20:54
Also, you're using item-1 when your explanation and expected output both suggest you mean item+1 — Kyle Willmon
– Kyle Willmon, Commented May 2, 2019 at 20:56
Please do not deface the original post because it makes the existing comments and answer/s non-understandable. You can instead put your explanation as a comment or as an answer. — Gino Mempin
– Gino Mempin, Commented May 3, 2019 at 9:39

chepner · Accepted Answer · 2019-05-02 21:26:09Z

5

You want to open the output file before the loop, and call write in the loop.

with open("input.bin", "rb") as binary_file:
    data = binary_file.read()

with open("output.bin", "wb") as binary_file2:
    binary_file2.write(bytes(item - 1 for item in data))

edited May 2, 2019 at 21:26

answered May 2, 2019 at 20:54

chepner

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2 Comments

chepner Over a year ago

bytes is, I think, a little asymmetrical; it's easy to get an int from a bytes, but less easy to turn a single int back into a bytes object.

chepner Over a year ago

The update shows a relatively efficient way to process multiple bytes; the struct module is another option.

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