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limitations of the future module and differences between Py2 and Py3 that are not (yet) handled

The following attributes on functions in Python 3 are not provided in Python 2.7:

__func__: see six.get_method_function() __self__: see six.get_method_self() __self__.__class__

Limitations of the futurize script

The futurize script is not yet mature; like 2to3, on which it is based, it makes mistakes. Nevertheless, it should be useful for automatically performing a lot of the repetitive code-substitution tasks when porting from Py2 to Py2/3.

Some new Python 3.3 features that cause SyntaxErrors on earlier versions are not currently handled by the futurize script. This includes:

  • yield ... from syntax for generators in Py3.3
  • raise ... from syntax for exceptions. (This is simple to fix manually by creating a temporary variable.)

Also:

  • Usage of file('myfile', 'w') as a synonym for open doesn't seem to be converted currently.

  • isinstance(var, basestring) should sometimes be converted to isinstance(var, str) or isinstance(var, bytes), or sometimes simply isinstance(var, str), depending on the context. Currently it is always converted to isinstance(var, str).

  • Caveats with bytes indexing!:

    b'\x00'[0] != 0
b'\x01'[0] != 1

    futurize does not yet wrap all byte-string literals in a bytes() call. This is on the to-do list. See :ref:`bytes-object` for more information.

Notes

  • Ensure you are using new-style classes on Py2. Py3 doesn't require inheritance from object for this, but Py2 does. futurize --from3 adds this back in automatically, but ensure you do this too when writing your classes, otherwise weird breakage when e.g. calling super() may occur.