(original) (raw)
On Sat, Feb 21, 2009 at 09:17, Jean-Paul Calderone <exarkun@divmod.com> wrote:
But that doesn't provide a clear way, short of screwing with sys.modules, to get at just the pure Python implementation for testing when the extensions are also present. The key point in trying to figure this out is to facilitate testing since the standard library already uses the import \* trick in a couple of places.
-Brett
If pickle and \_pypickle are both Python modules, and \_pypickle.A is intendedOn Fri, 20 Feb 2009 13:45:26 -0800, Brett Cannon <brett@python.org> wrote:
On Fri, Feb 20, 2009 at 12:53, Aahz <aahz@pythoncraft.com> wrote:
On Fri, Feb 20, 2009, Brett Cannon wrote:
> On Fri, Feb 20, 2009 at 12:37, Brett Cannon <brett@python.org> wrote:
>> On Fri, Feb 20, 2009 at 12:31, Daniel Stutzbach <
>> daniel@stutzbachenterprises.com> wrote:
>>>
>>> A slight change would make it work for modules where only key functions
>>> have been rewritten. For example, pickle.py could read:
>>>
>>> from \_pypickle import \*
>>> try: from \_pickle import \*
>>> except ImportError: pass
>>
>> True, although that still suffers from the problem of overwriting things
>> like \_\_name\_\_, \_\_file\_\_, etc.
>
> Actually, I take that back; the IMPORT\_STAR opcode doesn't pull in
anything
> starting with an underscore. So while this alleviates the worry above, it
> does mean that anything that gets rewritten needs to have a name that
does
> not lead with an underscore for this to work. Is that really an
acceptable
> compromise for a simple solution like this?
Doesn't \_\_all\_\_ control this?
If you define it, yes.
But there is another issue with this: the pure Python code will never call
the extension code because the globals will be bound to \_pypickle and not
\_pickle. So if you have something like::
# \_pypickle
def A(): return \_B()
def \_B(): return -13
# \_pickle
def \_B(): return 42
# pickle
from \_pypickle import \*
try: from \_pickle import \*
except ImportError: pass
If you import pickle and call pickle.A() you will get -13 which is not what
you are after.
to be used all the time, regardless of whether \_pickle is available, then
there's not really any reason to implement A in \_pypickle. Just implement it
in pickle. Then import whatever optionally fast thing it depends on from
\_pickle, if possible, and fall-back to the less fast thing in \_pypickle
otherwise.
This is really the same as any other high-level/low-level library split. It
doesn't matter that in this case, one low-level implementation is provided as
an extension module. Importing the low-level APIs from another module and
then using them to implement high-level APIs is a pretty common, simple,
well-understood technique which is quite applicable here.
But that doesn't provide a clear way, short of screwing with sys.modules, to get at just the pure Python implementation for testing when the extensions are also present. The key point in trying to figure this out is to facilitate testing since the standard library already uses the import \* trick in a couple of places.
-Brett