Bytes not the same when converting from bytes to string to bytes

Question

I have the following problem. When I read the bytes of an file via with open(r'file-path', 'rb') as file: data = file.read() and write this byte object "data" into in other file using 'wb'. The out coming file is fine and there are no problems. But when I first convert the byte object "data" into a string data_str = str(data) and than convert it back using new_data = data_str.encode() "new_data" looks exactly the same as "data", but if I write "new_data" into a file the file isn´t working. Or as in my case not recognized as being an mp3 file, even though the bytes are the same on the first look.

I know it seems useless to convert the bytes into a string and than back into bytes, but in my case I really have to do that.

Try printing data_str. You'll see that it has literal b'...' around it. That's not the same as the original data. str(data) is returning the printed representation of the byte string, not a string containing the bytes themselves. — Barmar
– Barmar, Commented Apr 15, 2022 at 21:09
As you describe, data != new_data. str() is NOT the way to convert bytes to strings. Why do you think you need convert to a string? Bytes strings are for binary data. Unicode strings are for text. If you need the binary data as text, you may need a binary to text converter (uuencode, base64, hexdump). This is an XY problem. Describe why you need to do this. — Mark Tolonen
– Mark Tolonen, Commented Apr 15, 2022 at 21:13

Barmar · Accepted Answer · 2022-04-15 21:13:55Z

1

data_str.encode() expects data_str to be the result of data.decode().

str(data) doesn't return a decoded byte string, it returns the printed representation of the byte string, what you would type as a literal in a program. If you want to convert that back to a byte string, use ast.literal_eval().

import ast

with open(r'file-path', 'rb') as file: 
    data = file.read()
    str_data = str(data)
    new_data = ast.literal_eval(str_data)
    print(new_data == data)

answered Apr 15, 2022 at 21:13

Barmar

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