What are the main differences between Python 2 and Python 3?

Main differences between Python 2 and Python 3:

• String handling:

  • Python 2: Strings are byte sequences by default (str). Unicode is represented by the unicode type.
  • Python 3: Strings are Unicode by default (str). Byte strings are represented by the bytes type.

• print operator:

  • Python 2: print is a statement using a keyword.
  • Python 3: print is a function.

  # Python 2
  print "Hello, World!"
  
  # Python 3
  print("Hello, World!")
  

• Integer division:

  • Python 2: The / operator performs integer division between integers.
  • Python 3: The / operator performs true division, returning a float. For integer division, use //.

  # Python 2
  print 5 / 2  # Outputs 2
  
  # Python 3
  print(5 / 2)  # Outputs 2.5
  print(5 // 2) # Outputs 2
  

• Exceptions:

  • Python 2: Syntax except exc, var:.
  • Python 3: Syntax except exc as var:.

  # Python 2
  try:
      # Code that may raise an exception
      pass
  except ValueError, e:
      print e
  
  # Python 3
  try:
      # Code that may raise an exception
      pass
  except ValueError as e:
      print(e)
  

• range() and xrange() functions:

  • Python 2: range() returns a list, xrange() returns a generator (iterator).
  • Python 3: range() always returns a generator (iterator), xrange() is removed.

• Renaming of some modules: For example, httplib to http.client, urllib2 to urllib.request and urllib.error.

• Changes in dict methods: dict.iterkeys(), dict.items(), and dict.values():

  • Python 2: These methods return lists.
  • Python 3: These methods return view objects (iterators).

• New-style classes:

  • Python 2: Must explicitly inherit from object to create new-style classes.
  • Python 3: All classes are new-style classes by default (inherit from object implicitly).

Many changes in Python 3 aimed to improve consistency, readability, and performance, as well as better Unicode handling.