[Python-Dev] [RELEASED] Python 2.7 alpha 2 (original) (raw)

"Martin v. Löwis" martin at v.loewis.de
Wed Jan 13 20:33:24 CET 2010

Note that increased 3.x compatibility in the most recent 2.x release will always help in two scenarios:

1. New projects that want to use 2.x only libraries but want to be ready for the Py3k transition in their own code (e.g. new 2.7 features like set literals, dict and set comprehensions and the view* dict methods can be translated far more cleanly by 2to3 than the closest comparable 2.6 code).

Of these, I can only see this being the case of the view* methods.

Why would set literals allow for code that more cleanly translates to 3.x? Suppose you write

f = set((1,2,3))

in 2.6. That code will work exactly the same way in 3.1, no translation needed at all. Likewise for dict and set comprehensions: the code is as cleanly translated in the 2.7 form as it is in any prior form (ie. no modifications).

As for view* methods: there is one important exception where the 2.6 equivalent is as cleanly translated as a view method:

for key in D: action

This is a more modern equivalent for iterating over D.keys(), so if your code still does the latter, just change it to the former (requires Python 2.2). I'd claim that this is the dominant case of traversing a dictionary (probably followed by iterating over .items()). So while it is true that only view* can be transformed correctly, I'd claim that this is an infrequent issue.

2. Similarly new projects that use a 3.x trunk can be more readily translated to a 2.7 version with 3to2, whereas some constructs may be difficult to recreate in earlier Python versions.

That may be true, but is besides the point here: the issue was whether a 2.8 release might help in migrating to 3.x. People who follow this approach already have 3.x code. Whether they would rather port to 2.8 only, and get that work with little effort, or whether they would port to 2.5 and later, and put in more work, I don't know - I guess we would have to ask these people. I would expect that they prefer if 2.x died rather sooner than later, because they can then stop supporting it.

Regards, Martin

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