A presentation to the West Yorkshire Python user group on moving from Python 2 to Python 3.

1. Moving To Python 3 Why? When? How? Nick Efford (http://about.me/nickefford)

2. Backwards Compatibility An ‘unwritten law’ of software... ...and yet Python 3 breaks it – so badly that “Hello World!” no longer works! $ python3 hello.py File "hello.py", line 1 print 'Hello World!' ^ SyntaxError: invalid syntax Why?

3. Compatibility Isn’t Free... Java is bloated because obsolete features are never removed

4. Python’s Brave Decision "The language has two choices: either continue to bear the burden of what are now considered poor design decisions... or suck it up and let us try and fix some of these problems. It's like going to the dentist; it may hurt, but if that minor toothache goes untreated and develops into an abscess, you will wish you were dead." – blog entry by Collin Winter

6. Maximal separation of concerns

7. Minimal surprise

8. Conceptual consistency

9. Appropriate complexity

10. Redundancy: Iteration in Java for (int i = 0; i < message.length(); ++i) { System.out.println(message.charAt(i)); } for (int i = 0; i < messageChars.length; ++i) { System.out.println(messageChars[i]); } 2 different ‘classic’ syntaxes for iterating over strings & arrays...

11. Redundancy: Iteration in Java for (int i = 0; i < vec.size(); ++i) { System.out.println(vec.elementAt(i)); } for (int i = 0; i < vec.size(); ++i) { System.out.println(vec.get(i)); } Enumeration<Integer> enumerator = vec.elements(); while (enumerator.hasMoreElements()) { System.out.println(enumerator.nextElement()); } Iterator<Integer> iterator = vec.iterator(); while (iterator.hasNext()) { System.out.println(iterator.next()); } ...+ 4 for vectors...

12. Redundancy: Iteration in Java for (int number : vec) { System.out.println(number); } for (char character : messageChars) { System.out.println(character); } ...+ 2 newer approaches for vectors & arrays...

13. Redundancy: Iteration in Java // Before JDK 1.5 BufferedReader inputFile = new BufferedReader( new FileReader(&quot;foo.txt&quot;)); String line = inputFile.readLine(); while (line != null) { System.out.println(line); line = inputFile.readLine(); } // Since JDK 1.5 Scanner inputFile = new Scanner(new File(&quot;foo.txt&quot;)); while (inputFile.hasNextLine()) { System.out.println(inputFile.nextLine()); } ...+ 2 for text files = 10 different iteration syntaxes!

14. Redundancy: Iteration in Python for character in string: print character for number in numbers: print number input_file = open('foo.txt') for line in input_file: print line, One syntax works for strings, lists and text files!

15. Redundancy: Integer Representation Java: four primitive integer types, four corresponding wrapper classes, BigInteger class Python 2: two integer types – int & long Python 3: one integer type – int

16. Redundancy: Object Model Two types of class in Python 2: class Foo: ... class Bar(object): ... Only ‘new-style’ classes exist in Python 3 (either syntax can be used) ‘old-style’ class ‘new-style’ class

17. Redundancy: Console Input Python 2 has two functions providing console input: raw_input , yielding input as a string input , yielding the result of calling eval on input Python 3 has one function, input , with the same behaviour as Python 2’s raw_input (New programmers also find this less surprising...)

19. Greater flexibility provided by keyword arguments print(text, file=output_file) print(x, y, z, sep=':')

20. Minimal Surprise: Integer Division >>> 5 / 3 1.6666666666666667 >>> 5 // 3 1 Python 3 >>> 5 / 3 1 Python 2 (like C, C++, Java – but unlike proper arithmetic)

21. Separation of Concerns: Text vs Binary Data Python 2: str for ASCII text strings & byte strings unicode for Unicode strings Two representations of text, overlapping with one for binary data! Python 3: str for Unicode text strings bytes for strings of bytes Clean separation of text and binary representations, with encode & decode methods for conversion

23. Is less surprising

24. Is more consistent

25. Separates concerns more cleanly

27. Development of the Python 2 line has come to an end with Python 2.7 (apart from bug fixes)

28. Focus for future innovation will be Python 3 (notwithstanding Jython, IronPython, PyPy, etc)

30. Converts Python 2 code to Python 3

31. Integrates with D istutils and can run during installation, allowing you to support 2 & 3 from one codebase

32. Cannot do a perfect job; you may need more unit tests and may have to refactor your code a bit

33. Only handles Python code; C extensions need to be converted to Python 3 API by hand

35. Availability of books, online tutorials, etc

36. Running multiple Python versions alongside each other is a pain ( virtualenv helps a lot)