Python 3.1 Features

Much as Python 2.6 incorporated features from Python 3.0, version 2.7 incorporates some of the new features in Python 3.1. The 2.x series continues to provide tools for migrating to the 3.x series.

A partial list of 3.1 features that were backported to 2.7:

• The syntax for set literals ({1,2,3} is a mutable set).
• Dictionary and set comprehensions ({i: i*2 for i in range(3)}).
• Multiple context managers in a single with statement.
• A new version of the io library, rewritten in C for performance.
• The ordered-dictionary type described in PEP 372: Adding an Ordered Dictionary to collections.
• The new "," format specifier described in PEP 378: Format Specifier for Thousands Separator.
• The memoryview object.
• A small subset of the importlib module, described below.
• The repr() of a float x is shorter in many cases: it’s now based on the shortest decimal string that’s guaranteed to round back to x. As in previous versions of Python, it’s guaranteed that float(repr(x)) recovers x.
• Float-to-string and string-to-float conversions are correctly rounded. The round() function is also now correctly rounded.
• The PyCapsule type, used to provide a C API for extension modules.
• The PyLong_AsLongAndOverflow() C API function.

Other new Python3-mode warnings include:

• operator.isCallable() and operator.sequenceIncludes(), which are not supported in 3.x, now trigger warnings.
• The -3 switch now automatically enables the -Qwarn switch that causes warnings about using classic division with integers and long integers.