[Python-Dev] Assertion in _PyManagedBuffer_FromObject() (original) (raw)

Stefan Krah stefan at bytereef.org
Sat Mar 3 12:08:38 CET 2012

Stefan Behnel <stefan_ml at behnel.de> wrote:

> 1. assert() is the wrong tool for this job

Absolutely.

I disagree. This assert() is meant for extension authors and not end users. I can't see how a reasonable release procedure would fail to trigger the assert().

My procedure as a C extension author is to test against a new Python version and then set the PyPI classifier for that version.

If I download a C extension that doesn't have the 3.3 classifier set, then as a user I would not be upset if the extension throws an assert or, as Thomas Wouters pointed out, continues to work as before if not compiled in debug mode.

> 2. the current check is too strict (it should just check for obj != > NULL, not obj == &exporter)

I don't know. The documentation isn't very clear on the cases where obj may be NULL. Definitely on error, ok, but otherwise, the bfgetbuffer() docs do not explicitly say that it must not be NULL (they just mention a "standard" case): http://docs.python.org/dev/c-api/typeobj.html#buffer-object-structures

How about this:

"The value of view.obj is the equivalent of the return value of any C-API function that returns a new reference. The value must be NULL on error or a valid new reference to an exporting object.

For a chain or a tree of views, there are two possible schemes:

  1. Re-export: Each member of the tree pretends to be the exporting object and sets view.obj to a new reference to itself.

  2. Redirect: The buffer request is redirected to the root object of the tree. Here view.obj will be a reference to the root object."

I think it's better not to complicate this familiar scheme of owning a reference by allowing view.obj==NULL for the general case.

view.obj==NULL was introduced for temporary wrapping of ad-hoc memoryviews via PyBuffer_FillInfo() and now also PyMemoryView_FromMemory().

That's why I explicitly wrote the following in the documentation of PyBuffer_FillInfo():

"If this function is used as part of a getbufferproc, exporter MUST be set to the exporting object. Otherwise, exporter MUST be NULL."

Stefan Krah

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