Python 3

This is a short summary of the major changes between the Python 2.x versions and Python 3.0, the latest version. Not all changes to the language are mentioned here; more comprehensive information can be found at the official Python web site or the “What’s New in Python 3.0” page. Some of the information here has already been talked about previously in this book; it is mentioned here again for easier reference.

• The print statement has been replaced with a print() function, with keyword arguments to replace most of the special syntax of the old print statement.
• Certain APIs don’t use lists as return types but give views or iterators instead. For example, the dictionary methods dict.keys(), dict.items(), and dict.values() return views instead of lists.
• Integers have only one type now (“long” integers for those who know them in other languages)
• The default value for division (e.g. 1/2) is to return a float value. To have truncating behaviour, i.e. return just the whole number, you must use a double forward slash: 1//2.
• All text is Unicode; 8-bit strings are no longer.
• “!=” is the only way to state “not equal to”; the old way (“<>”) has been removed.
• Calling modules from within functions is essentially gone. The statement “from module import *” can only be used at the module level, not in functions.
• String formatting has been changed, removing the need for the “%” formatting operator.
• The exception APIs have been changed to clean them up and add new, powerful features.
• raw_input() has been changed to just input().
• The C language API has been modified, such as removing support for several operating systems.

If you need to port Python 2.x code to version 3.0, there is a simple utility called “2to3” that will automatically correct and modify the legacy code to the new version. Generally speaking, you simply call the 2to3 program and let it run; it is robust enough to handle the vast majority of code changes necessary. However, the library is flexible enough to allow you to write your own “fixer” code if something doesn’t work correctly.