Enabling Future Language Features

Changes to the language that may potentially break existing code are introduced gradually. Initially, they appear as optional extensions, which are disabled by default. To turn on such extensions, use a special import statement of this form:

from __future__ import featurename

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This statement should generally appear at the top of a module file (possibly after a docstring), because it enables special compilation of code on a per-module basis. It’s also possible to submit this statement at the interactive prompt to experiment with upcoming language changes; the feature will then be available for the rest of the interactive session.

For example, in prior editions of this book, we had to use this statement form to demonstrate generator functions, which required a keyword that was not yet enabled by default (they use a featurename of generators). We also used this statement to activate 3.0 true division in Chapter 5, 3.0 print calls in Chapter 11, and 3.0 absolute imports for packages in Chapter 23.

All of these changes have the potential to break existing code in Python 2.6, so they are being phased in gradually as optional features enabled with this special import.