convert ascii to binary and write to a binary file - PYTHON 3.x

Question

I want to read binary file (like .bmp or .jpg) and convert each binary data to ascii and do multiplication (like ascii * 2) and convert back to binary format and write to a new file.

I wrote the program as below

    filename = input("enter file to read: ")
    readfile = filename + ".bmp"
    writefilebmp = filename + '1' + ".bmp"
    fr = open(readfile,"rb");
    fwbmp = open(writefilebmp,"wb");
    bytes_read = fr.read(1024)
    for b in bytes_read:
        print(b,end='')
        data = b*2;
        fwbmp.write(data)
    fr.close()
    fwbmp.close()

But i'm getting the below error

TypeError: 'int' does not support the buffer interface

Note: Once i the solution, i will modify the code to skip the bmp or jpeg header

Modified

    filename = input("enter file to read: ")
    readfile = filename + ".bmp"
    writefilebmp = filename + '1' + ".bmp"
    fr = open(readfile,"rb");
    fwbmp = open(writefilebmp,"wb");
    bytes_read = fr.read(20)
    for b in bytes_read:
        print(b,end='')
        data = b*2;
        fwbmp.write(bytes(data))
    fr.close()
    fwbmp.close()

i do have a requirement where all the pixels are very near to black (max of 20), i need to do pixel doubling on this... — Sathya
– Sathya, Commented Oct 23, 2013 at 11:35
And this isn't something that'd be easier by using Pillow? (bearing in mind you're dealing with image files and pixel data...) — Jon Clements
– Jon Clements, Commented Oct 23, 2013 at 11:37
I changed my program as above (modified), i see only 0's in the output file — Sathya
– Sathya, Commented Oct 23, 2013 at 11:44
Please give up this approach - look at image manipulation libraries as I've already said. It may take an hour to get to grips with one, but you've already wasted 41mins so far on this which will go nowhere... — Jon Clements
– Jon Clements, Commented Oct 23, 2013 at 11:50

Martijn Pieters · Accepted Answer · 2013-10-23 11:59:07Z

2

You are looping over bytes, which gives you integers. Convert back to bytes to write to a binary file:

fwbmp.write(bytes([data % 256]))

The modulo 256 is required here to limit the resulting int value to a permissible value. You could also cap the value at 255 with min():

fwbmp.write(bytes([min(data, 255)]))

or you could write two bytes if your doubling results in an overflow:

if data > 255:
    fwbmp.write(bytes(divmod(data, 256)))
else:
    fwbmp.write(bytes([data]))

or always produce 2 bytes regardless, where the first byte is either 0x01 or 0x00:

fwbmp.write(bytes(divmod(data, 256)))

You'll need to decide what you want your 'doubling' to mean in terms of output.

edited Oct 23, 2013 at 11:59

answered Oct 23, 2013 at 11:11

Martijn Pieters

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6 Comments

Jon Clements Over a year ago

They could, but then if they want to multiple by 2, 254 * 2 is not 255 nor 252... I'm not sure the OP actually wants to do anything like this at all... :P

Jon Clements Over a year ago

The later is more likely - which came a little while after my initial comment - however, then we're getting to struct.pack or int.to_bytes if possible... And good luck to anyone reading back what might be one byte or two byte :)

Sathya Over a year ago

@MartijnPieters i tried, fwbmp.write(bytes(data)) (since my data won't go beyond 256), but in the output file, i see only NULL

Martijn Pieters Over a year ago

@Sathya: bytes(0) will writes NULLs, yes. Are you certain your input is not null as well?

Martijn Pieters Over a year ago

@Sathya: ah, bytes(0) is an empty bytestring, bytes([0]) is one byte hex value 00. Corrected.

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