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Python 3.0

Python 3.0 is the first version of Python to break backward compatability with all previous Python versions. No matter how you program, the language changes in 3.0 - whether in syntax or the library - make it highly unlikely that any substantial amount of 2.x code will run on Python 3.0. Here you can learn how Python 3.0 differs from the versions before it.
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2 Articles in: Learn Python - Getting Started With Python 3.0
When to Use Text or Data in Python 3.0
The difference between text and data is critical for programming in Python 3.0. Understanding this all-pervasive difference and how it relates to your programming will save you a lot of debugging time.
Guido's Detailed Guide to Changes in 3.0
With the publication of Python 3.0, Guido van Rossum published "What's New in Python 3.0?" Intended as an overview, this article covers a lot more ground than most Python programmers will need. [This is likely to ensure that the various niches of the broad spectrum of Python coders are not left out.] It also tends to be a bit terse. Nonetheless, if you cannot find something mentioned in the guides on this site, chances are that it is covered here - possibly to a higher level than you wanted to go.