[Cython] star-imports in Cython/Includes/cpython considered harmful

Stefan Behnel stefan_ml at behnel.de
Wed Feb 22 08:13:32 CET 2012

Robert Bradshaw, 21.02.2012 21:14:
> On Mon, Feb 20, 2012 at 12:45 AM, Stefan Behnel wrote:
>> I just noticed that the star-imports in the cpython package have serious
>> side effects. The cpython/__init__.pxd file has this in it:
>> """
>> from cpython.version cimport *
>> from cpython.ref cimport *
>> from cpython.exc cimport *
>> from cpython.module cimport *
>> from cpython.mem cimport *
>> ...
>> """
>> This means that a direct cimport of any of those recursively cimported
>> names from the cpython package namespace will always trigger a complete
>> cimport of all cpython.* modules, thus
>> 1) wasting compile time
> Which is still pretty cheap (and should be made extremely cheap for
> unchanging pxd files).
>> 2) polluting the cpython package namespace
> It's neither inspected (by the end user) at runtime, nor in danger of
> conflicts, so I don't see this as a big deal. "Flat is better than
> nested."
>> 3) polluting the internal state of Cython's list of cimported modules
> This is really (1), other than that it doesn't really hurt to have
> extra keys in a dict.

Except when it has side-effects as in 4). If we can figure out what parts
of the cimported declarations are actually used, we could still drop
unnecessary declarations. But that doesn't currently happen.

>> 4) leading to things being defined in the generated C code that don't need
>> to be there, specifically the object structs of PyBoolObject and
>> PyComplexObject, which we provide definitions for in cpython.bool and
>> cpython.complex.
> This is an actual issue that should be fixed, and avoiding emitting
> unused code is a worthwhile goal in many contexts.

This issue was the reason I brought this up in the first place.

>> The point where I noticed this is when I got test errors in PyPy because it
>> doesn't expose the bool/complex structs. That wasn't because the test
>> actually used them, it was purely because it had this cimport in it:
>>  from cpython cimport PyObject
>> If it had used this instead:
>>  from cpython.object cimport PyObject
>> Cython would still have parsed all of those .pxd files, but at least it
>> wouldn't have used those declarations to do any harm.
>> Now, I'm 100% certain that there is user code out there that does this as
>> well. Fixing this bug by removing all the star-imports will break that
>> code, but keeping the cimports in the package will trap the average lazy
>> user into cimporting stuff from there directly.
>> I'll add a big warning to the package for now, but I wonder if we shouldn't
>> accept breaking this code at some point (and sooner is always better than
>> later).
> I actually don't think it's a bad thing to allow people to be "lazy"
> and import directly from cpython rather than have to hunt down the
> specific subpackages that things live in. In fact, it's cleaner in
> some sense to not have to know about the subdivisions of the cpython
> library. (It's like including "Python.h" rather than all the
> individual header files.)

As long as it doesn't have side-effects to do this, it's not a problem. And
I agree that it's better to actually fix the side-effects. Just not
importing from the cpython package doesn't make the problem go away that
your code won't compile on PyPy as soon as you cimport anything from
cpython.bool or cpython.complex.


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