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Thu Jul 25 12:41:35 CEST 2013 · https://mail.python.org/pipermail/python-dev/2013-July/127562.html

On 07/25/2013 11:45 AM, Maciej Fijalkowski wrote:
> On Wed, Jul 24, 2013 at 7:22 PM, Laurent Gautier <lgautier at gmail.com> wrote:
>> On 07/24/2013 06:30 PM, Chris Angelico wrote:
>>> On Thu, Jul 25, 2013 at 2:21 AM, Laurent Gautier <lgautier at gmail.com>
>>> wrote:
>>>> - errors that are typical of "Python 2 script running with Python
>>>> 3"-specific are probably limited (e.g., use of unicode, use of xrange,
>>>> etc...)
>>>>
>>> The most common, in interactive scripts at least, is likely to be:
>>>
>>>>>> print "Hello, world!"
>>> SyntaxError: invalid syntax
>>>
>>> How helpful it's possible to make that one, I don't know. Is it safe
>>> to presume that it's more likely a syntax error will come from an
>>> interpreter version mismatch than a code bug?
>>
>> The wrapper in /usr/bin/python:
>> - could use what is in 2to3. I think that most of the cases are solved
>> there.
>> - whenever interactive, could have an intermediate layer between the input
>> in the console and execution.
> So you suggest that instead of a clear SyntaxError you should end up
> with a confusing error (something has no attribute xyz or so) after a
> while (if say somone tries to load twisted via 2to3).

In a sense, yes.

I think that the a priori (expectations) a user has plays a role in 
whether something is confusing or not.
Consider the two following situations:
- a programmer is working on code, and is testing it. If the program 
fails, this is because he/she made a mistake and the error message 
should indicate where the error originates from
- a user is running a python script (he expects to be working), and is 
using the default /usr/bin/python (formerly Python 2, now Python 3). If 
the program fails because of obvious Python 2-only idioms, reporting 
this rather that the SyntaxError is much less confusing.

Having that said, the comments are pointing out that this is may be not 
a completely good idea (most notably because of how slow 2to3 is).
A wrapper producing an unconditional warning about the default python 
being changed to Python 3, and differences between Python 2 and Python 3 
the possible source of errors, (as I think it was suggested) is going to 
be a better idea.

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[Python-Dev] Python 3 as a Default in Linux Distros