[Python-Dev] Breaking undocumented API

[Tres Seaver]

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On 11/09/2010 11:12 PM, Stephen J. Turnbull wrote:
> Nick Coghlan writes:
> 
>  > > Module writers who compound the error by expecting to be imported
>  > > this way, thereby bogarting the global namespace for their own
>  > > purposes, should be fish-slapped. ;)
>  > 
>  > Be prepared to fish-slap all of python-dev then - we use precisely
>  > this technique to support optional acceleration modules. The pure
>  > Python versions of pairs like profile/_profile and heapq/_heapq
>  > include a try/except block at the end that does the equivalent of:
>  > 
>  >   try:
>  >     from _accelerated import * # Allow accelerated overrides
>  >   except ImportError:
>  >     pass # Use pure Python versions
> 
> But these identifiers will appear at the module level, not global, no?
> Otherwise this technique couldn't be used.  I don't really understand
> what Tres is talking about when he writes "modules that expect to be
> imported this way".  The *imported* module shouldn't care, no?  This
> is an issue for the *importing* code to deal with.

Right -- "private" star imports aren't the issue for me, because the
same user who creates them is responsible for the other end fo the
stick.  I was ranting about library authors who document star imports as
the expected usage pattern for their external users.

Note that I still wouldn't use star imports in the "private
acceleration" case myself.  I would prefer a pattern like:

- ----------------------- $< -----------------------------
# spam.py

# Pure python API implementation
def foo(spat, blarg):
    ...

def bar(qux):
    ...

# Replace with accelearated C implemenataion
try:
    import _spam
except ImportError:
    pass # accelerated version not available
else:
    foo = _spam.foo
    bar = _spam.bar
- ----------------------- $< -----------------------------

This explicit name remapping catches unintentional erros (e.g., _spam
renames a method) better than the star import.


Tres.
- -- 
===================================================================
Tres Seaver          +1 540-429-0999          tseaver at palladion.com
Palladion Software   "Excellence by Design"    http://palladion.com
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