Byte file conversion via python 3

Question

Is there any simple way for me to read the contents of a binary file as a binary string, turn it into a normal (utf-8) string, do some operations with it, turn it back into a binary string and write it into a binary file? I tried doing something as simple as:

a_file = open('image1.png', 'rb')
text = b''
for a_line in a_file:
 text += a_line
a_file.close()
text2 = text.decode('utf-8')
text3 = text2.encode()
a_file = open('image2.png', 'wb')
a_file.write(text3)
a_file.close()

but I get 'Unicode can not decode bytes in position...'

What am I doing terribly wrong?

• Ignacio Vazquez-Abrams: Why do you think a PNG file would contain text?
• martineau: Not sure what you're trying to accomplish, but this answer to another question may help.

Answer 1

The utf8 format has enough structure that random arrangements of bytes are not valid UTF-8. The best approach would be to simply work with the bytes read from the file (which you can extract in one step with text = a_file.read()). Binary strings (type bytes) have all the string methods you'll want, even text-oriented ones like isupper() or swapcase(). And then there's bytearray, a mutable counterpart to the bytes type.

If for some reason you really want to turn your bytes into a str object, use a pure 8-bit encoding like Latin1. You'll get a unicode string, which is what you are really after. (UTF-8 is just an encoding for Unicode-- a very different thing.)

• ShadowRanger: And note, if you settle on a working encoding (e.g. latin-1), you don't need to handle the encode/decode yourself in Python 3. Just change open('image1.png', 'rb') to open('image1.png', 'r', encoding='latin-1'), and for the output, open('image2.png', 'w', encoding='latin-1') and you can read and write without bothering to manually encode/decode; it will have been decoded to str for you on read, and will encode the str for you on write.
• alexis: Good point; though opening the files in binary mode makes the code a little more transparent... I'm not sure the OP should be converting to str at all.