PEP suggestion: Uniform way to indicate Python language version

[Steven D'Aprano]

On Monday 22 August 2016 14:33, Chris Angelico wrote:

> On Mon, Aug 22, 2016 at 1:37 PM, rocky <rocky at gnu.org> wrote:
>> Sorry should have been:
>>
>>  assert sys.version_info >= (3,0)
> 
> The next question is: How common is code like this? I don't put
> version checks in any of my modules. Adding magic comments would be of
> value only if this sort of thing is common enough to need its own
> syntax.

Most of my code is supposed to work on Python 2.4 through the latest 3.x 
version. That means I have lots of version-specific code.

But most of it uses feature detection rather than version checks. So instead of 
writing:

if sys.version < '3':
    def next(iterator):
        return iterator.next()


I write this:

try:
    next
except NameError:
    def next(iterator):
        return iterator.next()

or possibly this:


if not hasattr(builtins, 'next'):
    def next(iterator):
        return iterator.next()


But some features depend on new syntax, and you can't check for the 
availability of new syntax at runtime. (Well, you can, by clever use of exec(), 
but its too much trouble than its worth.) So very, very occasionally I have to 
do a version check:


if sys.version < '3':
    import mymodule2 as mymodule
else:
    import mymodule3 as mymodule


But... I don't understand what this proposal actually is. We already have a 
uniform way to indicate the Python language version: check sys.version, or 
sys.version_info, or sys.hexversion, whichever is more appropriate for your 
needs. Because this is just an ordinary comparison, you can then do whatever 
you like, no matter how outlandish or unusual:


if sys.version < '3':
    try:
        import mymodule3
    except SyntaxError:
        print("Yay, a syntax error!")
    else:
        print("Now that's weird, that shouldn't happen...")
        del mymodule3
    import mymodule2 as mymodule
elif config[RUN_OLD_VERSION]:
    import mymodule2 as mymodule
else:
    import mymodule3 as mymodule


Could somebody (the OP?) please explain what is the purpose of this proposal, 
what it does, how it works, and when would people use it?



-- 
Steve