http://docs.python.org/3/howto/pyporting.html
http://docs.pythonsprints.com/python3_porting/py-porting.html

1. Exception capturing old:

try:
  raise Exception()
except Exception, exc:
  # Current exception is 'exc'.
  pass

Exception capturing new:

try:
  raise Exception()
except Exception as exc:
  # Current exception is 'exc'.
  # In Python 3, 'exc' is restricted to the block; in Python 2.6/2.7 it will "leak".
  pass

Compatible way:

try:
  raise Exception()
except Exception:
  import sys
  exc = sys.exc_info()[1]
  # Current exception is 'exc'.
  pass

2. Compatibly opening text files containing unicode strings: codecs.open()

3. Text literals should have the u prefix

4. Umbenennungen

ConfigParster -> configparser
Tkinter -> tkinter
tkFileDialog -> tkinter.filedialog
tkFont -> tkinter.font
tkMessageBox -> tkinter.messagebox

try:
  import OLDNAME
except ImportError:
  import NEWNAME

5. print Inkompatibel

Python 2:
print 5,6  ->    5 6
print(5,6) ->    (5,6)

Python 3:
print 5,6 -> ERROR
print(5,6) -> 5 6
print((5,6)) -> (5,6)

6. sets deprecated since 2.6
(set and frozenset are builtin types since 2.4)

Solution: rename Set to set, ImmutableSet to frozenset, and do at the beginning:

try:
   set # succeeds from 2.4
except NameError: # 2.3 or earlier
   from sets import Set as set, ImmutableSet as frozenset