This talks takes a look at the various options of migrating to Python 3, and takes up examples of some tricks you can do to make you code run unmodified under both 2.6 and 3.0. Video at http://pycon.blip.tv/file/1949281/

1. Python 2.6 and 3.0 Compatibility
Lennart Regebro
http://regebro.wordpress.com/
regebro@gmail.com
PyCon 2009, Chicago

2. Python 3.0 is here!

3. Python 3 is incompatible

4. Python 3 is incompatible
O RLY?

5. Python 3 is incompatible
Ya, really!
O RLY?

6. Strategies to deal with
incompatibility

7. For applications:
Just port to Python 3

8. 2to3 helps you port

9. Tests are really helpful

10. Libraries need to support both
Python 2 and Python 3

11. For most libraries:
Develop in Python 2 and run 2to3
for Python 3 support

12. 2to3 supported development
has a significant startup cost

13. For stable libraries:
Just port to Python 3

14. Platform Extensions/
Plugins/etc.

15. No Plone user will switch to
Python 3 until important
extensions are available

16. Nobody will make the extensions
support Python 3 until they
themselves move to Python 3

17. Dead lock

18. The Zope experience

19. Zope 3: A complete break

20. Zope 3.1 etc: No break

21. Wanted: Gradual upgrade path

22. First support 2.5 and 2.6,
then support 2.6 and 3.0,
finally dropping 2.x completely

23. Python 3 is incompatible
Oh, right, I forgot
already.

24. Python 2.6 introduces quite a lot
of forward compatibility!

25. Python 2.5:
except Exception, e:
Python 3.0:
except Exception as e:

26. Python 2.6:
Both works!

27. Python 2.5:
3/2 == 1
Python 3.0:
3/2 == 1.5

28. Solutions:
3//2 == 1
from __future__ import division
3/2 == 1.5

29. Python2.5:
print >> file, “bla”, “bla”, “bla”,
Python3.0:
print(“bla”, “bla”, “bla”,
file=file, ending='')

30. Python 2.6:
from __future__ import
print_function

31. Python2.5:
u”Üniçodê”
Python3.0:
“Üniçodê”

32. Python 2.6:
from __future__ import
unicode_literals

33. from __future__ import
unicode_literals
try:
str = unicode
except NameError:
pass
isinstance(type(“Üniçodê”), str)

34. Python2.5:
open('filename', 'r').read()
'A stringn'
open('filename', 'rb').read()
'A stringn'

35. Python3.0:
open('filename', 'r').read()
'A stringn'
open('filename', 'rb').read()
b'A stringn'

36. Python 2.6:
b“A string”[5] == “i”
Python 3.0:
b“A string”[5] == 105

37. Solutions for 2.6 and 3.0:
bytearray(b”A list of bytes”)
bytearray(open(file, “rb”).read())

38. Python 2.6:
>>> bytes(a_byte_array)
'Binary data'
Python 3.0:
>>> bytes(a_byte_array)
b'Binary data'

39. Python2.5:
open(“unicodefile”).read()
'xc3x9cnixc3xa7odxc3xaan'
Python3.0:
'Üniçodên'

40. Solution for 2.6 and 3.0:
infile =open(“unicodefile”, “rb”)
uni = infile.read().decode(enc)

41. try:
from cStringIO import StringIO
except ImportError:
from io import StringIO

42. Python2.5:
dict.items()/dict.keys()/dict.values()
return lists
Python3.0:
dict.items()/dict.keys()/dict.values()
return views

43. Python 2.5:
foo = bar.keys()
foo.sort()
Python 2.4 - 3.0:
foo = sorted(bar.keys())

44. Removed:
dict.iteritems()
dict.itervalues()
dict.iterkeys()

45. Python 2.6 solution:
try:
iter = d.iteritems()
except AttributeError:
iter = d.items()

46. Aaaahhhhh........
Now meta classes make sense

47. infile = open('something', 'rb')
bdata = infile.read()
python2.6:
pass
python 3.0:
sdata = bdata.decode()

48. Python 2.x and 3.0:
sdata = str(bdata.decode('ascii'))

49. Running on both 2.x and 3.0 will
mean some ugly hacks no matter
if you use 2to3 or not

50. Using 2to3:
Easy to support 2.3, 2.4, 2.5
Few contortions
Some setup cost
Single branch of code

51. Separate branches:
Easy to support 2.3, 2.4, 2.5
No contortions
No setup cost
You have to fix all bugs twice

52. 2.6 and 3.0 support without 2to3:
More contortions
Low setup cost
Single branch of code
But, no support for < 2.6

53. Supporting Python 2.5 or lower
and 3.0 without 2to3:
Contortion fest!
High hack value
Lot's of work

54. So what if you decide to go for
2.6 and 3.0 without 2.6?
1. 2to3
2. Make it run under 3.0
3. Backport to Python 2.6

55. 2.6 compatible 2to3?

56. Preparing yourself for 3.0

57. Already in 2.6:
String exceptions are gone
“as” and “with” are keywords

58. Python 2.3:
alist.sort(cmp=func)
Python 2.4 and later:
alist = sorted(alist, key=func)

59. Use // when you want integer
division.
Use
from __future__ import division
already now

60. Mark binary files with “rb” or
“wb”

61. http://code.google.com/p/python-incompatibility
regebro@gmail.com