2 vs 3

• print statement has been replaced with print() function.

  # python 2
  print "Hello World!"
  # python 3
  print("Hello World!")
  

• There is only one integer type left, int.
• Some methods such as map() and filter() return iterator objects in Python 3 instead of lists in Python 2.
• In Python 3, a TypeError is raised as warning if we try to compare unorderable types. e.g. 0 > None is no longer valid.
• Python 3 provides Unicode (utf-8) strings while Python 2 has ASCII str() types and separate unicode().
• A new built-in string formatting method format() replaces the % string formatting operator.
• In Python 3, we should enclose the exception argument in parentheses.

  # python 2
  raise IOError, "file error"
  # python 3
  raise IOError("file error")
  

• In Python 3, we have to use the as keyword now in the handling of exceptions.

  # python 2
  Try:
      ...
  except NameError, err:
      ...
  
  # python 3
  Try:
      ...
  except NameError as err:
      ...
  

• The division of two integers returns a float instead of an int. // can be used to have the old behavior.