This is modified from my answer at Stack Overflow: Python 3 TypeError: must be str, not bytes with sys.stdout.write().

Python 3 handles strings a bit different. Originally there was just one type for strings: str. When unicode gained traction in the ’90s the new unicode type was added to handle Unicode without breaking pre-existing code1. This is effectively the same as str but with multibyte support.

In Python 3 there are two different types:

In Python 2 implicitly assuming an encoding could cause a lot of problems; you could end up using the wrong encoding, or the data may not have an encoding at all (e.g. it’s a PNG image).
Explicitly telling Python which encoding to use (or explicitly telling it to guess) is often a lot better and much more in line with the “Python philosophy” of “explicit is better than implicit”.

This change is incompatible with Python 2 as many return values have changed, leading to subtle problems like this one; it’s probably the main reason why Python 3 adoption has been so slow. Since Python doesn’t have static typing2 it’s impossible to change this automatically with a script (such as the bundled 2to3).

Of course, UTF-8 may not be the correct character set in your case, so be sure to use the correct one.

In your specific piece of code, nextline is of type bytes, not str, reading stdout and stdin from subprocess changed in Python 3 from str to bytes. This is because Python can’t be sure which encoding this uses. It probably uses the same as sys.stdin.encoding (the encoding of your system), but it can’t be sure.

You need to replace:




or maybe:


You will also need to modify if nextline == '' to if nextline == b'' since:

>>> '' == b''

Also see the Python 3 ChangeLog, PEP 358, and PEP 3112.

  1. There are some neat tricks you can do with ASCII that you can’t do with multibyte character sets; the most famous example is the “xor with space to switch case” (e.g. chr(ord('a') ^ ord(' ')) == 'A') and “set 6th bit to make a control character” (e.g. ord('\t') + ord('@') == ord('I')). ASCII was designed in a time when manipulating individual bits was an operation with a non-negligible performance impact. 

  2. Yes, you can use function annotations, but it’s a comparatively new feature and little used. 


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