[Python-ideas] Experimental package

João Bernardo jbvsmo at gmail.com
Sun Mar 3 02:00:06 CET 2013

João Bernardo

2013/3/2 Terry Reedy <tjreedy at udel.edu>

> On 3/2/2013 4:04 PM, João Bernardo wrote:
>> I was thinking about "yield from" and how many times I wanted to use the
>> feature, but because of backward compatibilities, I couldn't.
> Load 3.3 and use it. If you need an external 3.x library that will not run
> on 3.3 yet, 6 months after release, bug the author. If you want your code
> to run on earlier releases, select new and old versions with 'if
> version....'/
>  It's sad that it may take possibly 5 or 10 years to see this statement
>> being used in real programs...
> But it will not take that long. 'yield from' plays an essential role in
> Guido's new asynch package (code name: Tulip), which he hope will be ready
> for 3.4. It will probably also run in 3.3, but that will be it. People who
> want to use it will have to upgrade.
Writing new stuff for the stdlib doesn't need to be compatible with older
python versions... I develop on 3.3 but need to support 3.1 or 3.0.

>  I don't know if this was proposed before, but why can't Python have an
>> "__experimental__" magic package for this kind of stuff?
> Experimental modules and packages are usually available on PyPI. I think
> experimental syntax that might be removed is a BAD idea. Getting rid of
> things that were not experimental is bad enough.
If something is experimental, people are advised against using it for
production. The idea here is to make forward compatibility possible.

> Note that*the idea is not for people to use experimental features* when
>> they appear, but to have a fallback for when the feature becomes official!
> The details of a new feature are not fixed until they are fixed, when it
> becomes official be being added. Even future imports are not backported as
> bugfix releases do not get new features.
It is not about backporting features. Think about the "with" statement:

It was added on version 2.5 (not as default) then it changed on version 2.7
to allow multiple contexts.
You can write code compatible with 2.5, 2.6 and 2.7 by just restricting
yourself to use only one context per block.

Now, if you had a partial version of "yield from" syntax on 3.1 that could
solve 50% of the problems of the current syntax, it would be used a lot by
The current problem is that on older versions, using it gives
*SyntaxError *that
cannot be bypassed by a try...except... statement!

Having the syntax available makes possible to do:

    if sys.version_info >= (3,3):
        yield from foo()
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