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 bytes 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. pasteurize 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.