Write Python version independent code: struct.pack

Question

Let's assume you have the following Python2 Code:

#!/usr/bin/env python
import struct
struct.pack('s', 'hello')

This works fine under Python2, but it won't run under Python3, as the implementation of struct.pack changed. struct.pack now expects a bytes object for a string identifier and not a string any more. The fix for running this code under Python3 would be something like: struct.pack('s', bytes('hello', 'utf-8')). But again, this code will not run under Python2 :-)

I'm trying to code version independent as I don't want to force anybody to install python2 or 3.

What would be the best approach for getting this piece of code version independent?

• Martijn Pieters: Python 2 str is a bytestring. It is not struct that changed, it is the Python string object. Python 2 unicode became str in Python 3, Python 2 str became bytes in Python 3.
• Martijn Pieters: So your problem is a generic how do I produce a bytestring in both Python 2 and 3 issue. b'hello' will work in both Python 2 and 3 if you are using literals.
• rralf: Well in fact I use a variable rather than a literal. But wait, look at the documentation of struct. It states that the python types changed from string to bytes.
• Martijn Pieters: Yes, because the type was renamed, basically.
• Martijn Pieters: Yes, and the Python 3 str type is the same thing as unicode in Python 2. You'd encode unicode values in Python 2 too.

Answer 1

The Python 2 str type was basically renamed to bytes in Python 3, while the unicode type became str. You are in essence sending bytestrings in Python 2 and Unicode strings in Python 3 to struct.pack().

It could be the correct fix is to use Unicode everywhere in both Python 2 and 3 (and thus always encode when using struct.pack(), regardless of the Python version).

The alternative is to use an isinstance test:

value = 'hello'
if not isinstance(value, bytes):
 vaule = value.encode('utf8')

This works in both Python 2 and 3, because in Python 2 bytes is an alias for str.