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Thu Nov 5 21:22:43 CET 2009 · https://mail.python.org/pipermail/python-dev/2009-November/093796.html

Martin v. Löwis wrote:
>> Mike Krell wrote:
>>     
>>>     Well, 3to2 would then be an option for you: use Python 3 as the source
>>>     language.
>>>       
>> Making life easier for 3to2 is an *excellent* rationale for backports.
>>
>>     
>
> I'm skeptical. If new features get added to 2.7: why would that simplify
> 3to2? It couldn't *use* these features, since it surely would have to
> support 2.6 and earlier as well.
>
> Not sure what 3to2 would do about difficult-to-convert 3.x feature (as,
> perhaps, the nonlocal keyword). If it currently gives up, it then may
> offer you to restrict your target versions to 2.7+. Not sure whether
> users would use that option, though - perhaps they rather stop using
> nonlocal in 3.x if 3to2 cannot support it for all 2.x versions they are
> interested in.
>   

I would have thought you could translate nonlocal with the following:

Python 3:

def scope():
    name = value
    do_something_with(name)
    def inner():
        nonlocal name
        name = new_value
        do_something_else(name)

Python 2

def scope():
    name = [value]
    do_something_with(name[0])
    def inner():
        name[0] = new_value
        do_something_else(name[0])

I would love to see nonlocal backported to 2.7 as it cleans up a simple 
pattern that I use relatively often for testing.

Suppose you have an class and you want to test that method a calls 
method b, in Python 2 you might write something like this:

def test_method_a_calls_method_b():
   instance = SomeClass()
   was_called = []
   def method_b():
      was_called.append(True)

   instance.method_b = method_b
   instance.method_a()

   assert was_called == [True]

in Python 3 you can replace this with the slightly nicer:

def test_method_a_calls_method_b():
   instance = SomeClass()
   was_called = False
   def method_b():
      nonlocal was_called
      was_called = True

   instance.method_b = method_b
   instance.method_a()

   assert was_called

As to the argument that releasing 2.7 is pointless as few people would 
use it for several years, the success of Python 2.6 shows that although 
*many* people don't / can't use new versions of Python for several years 
many other people are able to and do use new versions of Python.

All the best,

Michael Foord

> Perhaps 3to2 has a work-around that still provides a good backport in
> most cases. Then, the backport would not make the tool any simpler: if
> 3to2 would start using the backport, it would actually get more
> complicated (not easier), as it now needs to support two cases.
>
> Regards,
> Martin
> _______________________________________________
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>   

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[Python-Dev] 2.7 Release? 2.7 == last of the 2.x line?