Changes in data types and collections and string literals

Changes in how Python represents datatypes and collections require the most effort when the developer tries to maintain compatibility or simply ports existing code to Python 3. While incompatible syntax or standard library changes are easily noticeable and often easy to fix, changes in collections and types are either non-obvious or require a lot of repetitive work. The list of such changes is long and the official documentation is the best reference.

Still, this section must cover the change in how string literals are treated in Python 3, because it seems to be the most controversial and discussed change in Python 3, despite being a very good move that makes things more explicit.

All string literals are now Unicode, and bytestring literals require b or B prefix. For Python 3.0 and 3.1, the old Unicode u prefix (like u"foo") is illegal and will raise a syntax error. Dropping off that prefix was the main reason for most of the controversies. It made it really hard to create code compatible with different branches of Python—Python in version 2.x relied on these prefixes in order to create Unicode literals. This prefix was brought back in Python 3.3 to ease the integration process, although it now lacks any syntactic meaning.