python replace string method in read file as binary

Question

I opened an image file in readbinary("rb") mode and stored the data in a variable. Now i want to replace some values in the binary with my values.. but its not working using usual replace method of string

f=open("a.jpg","rb")
a=f.read()

''' first line is '\xff\xd8\xff\xe0\x00\x10JFIF\x00\x01\x01\x00\x00\x01\x00\x01\x00\x00\xff\xe1\x00*Exif\x00\x00II*\x00\x08\x00\x00\x00\x0 '''

a=a.replace("ff","z")
print a

#but there's no change in a

can anyone tell where iam going wrong.. i also tried

a=a.replace(b'ff',b'z')

but still the output was unchanged.

can anyone tell what iam supposed to do to perform the replacement?

Answer 1

I don't know which version of Python you're using (this kind of operations are different between 2 and 3), but try a = str(a) before executing replace method.

EDIT: For python 2.7 only reasonable way I've discovered to do what you want is use built-in function repr . Example:

>>> picture = open("some_picture.jpg", 'rb')
>>> first_line = picture.readline()
>>> first_line
'\xff\xd8\xff\xe0\x00\x10JFIF\x00\x01\x01\x00\x00\x01\x00\x01\x00\x00\xff\xe1\x00*Exif\x00\x00II*\x00\x08\x00\x00\x00\x01\x001\x01\x02\x00\x07\x00\x00\x00\x1a\x00\x00\x00\x00\x00\x00\x00Google\x00\x00\xff\xdb\x00\x84\x00\x03\x02\x02\x03\x02\x02\x03\x03\x03\x03\x04\x03\x03\x04\x05\x08\x05\x05\x04\x04\x05\n'
>>> repr(first_line)
>>> "'\\xff\\xd8\\xff\\xe0\\x00\\x10JFIF\\x00\\x01\\x01\\x00\\x00\\x01\\x00\\x01\\x00\\x00\\xff\\xe1\\x00*Exif\\x00\\x00II*\\x00\\x08\\x00\\x00\\x00\\x01\\x001\\x01\\x02\\x00\\x07\\x00\\x00\\x00\\x1a\\x00\\x00\\x00\\x00\\x00\\x00\\x00Google\\x00\\x00\\xff\\xdb\\x00\\x84\\x00\\x03\\x02\\x02\\x03\\x02\\x02\\x03\\x03\\x03\\x03\\x04\\x03\\x03\\x04\\x05\\x08\\x05\\x05\\x04\\x04\\x05\\n'"
>>> repr(first_line).replace('ff', 'SOME_OTHER_STRING')
"'\\xSOME_OTHER_STRING\\xd8\\xSOME_OTHER_STRING\\xe0\\x00\\x10JFIF\\x00\\x01\\x01\\x00\\x00\\x01\\x00\\x01\\x00\\x00\\xSOME_OTHER_STRING\\xe1\\x00*Exif\\x00\\x00II*\\x00\\x08\\x00\\x00\\x00\\x01\\x001\\x01\\x02\\x00\\x07\\x00\\x00\\x00\\x1a\\x00\\x00\\x00\\x00\\x00\\x00\\x00Google\\x00\\x00\\xSOME_OTHER_STRING\\xdb\\x00\\x84\\x00\\x03\\x02\\x02\\x03\\x02\\x02\\x03\\x03\\x03\\x03\\x04\\x03\\x03\\x04\\x05\\x08\\x05\\x05\\x04\\x04\\x05\\n'"

Answer 2

When you display a string at the Python console, the string is encoded so that you can see all of the characters, even the ones that aren't printable. Whenever you see something like \\xff , that's not 4 characters, it's a single character in hex notation. To replace it, you also need to specify the same single character.

a = a.replace("\xff", "z")