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But only Cython does not set suboffsets to NULL and you already have a small 
patch to fix that.
The Python 3 docs say "suboffsets only if needed" and the PEP says the same,
so the situation is not completely undocumented.
I think your doc patch goes too far. Buffers propagate in unpredictable ways, 
and since the original consumer request is *not stored* in the buffer, setting
unneeded fields to NULL provides a way to figure out the original request.
It is actually *an optimization* to set suboffsets to NULL: Compliant code
that uses arbitrary buffers always has to check for:
 if (suboffsets != NULL && suboffsets[i] >= 0)
 ...
If suboffsets are consistently NULL, at least hopefully you get branch 
prediction to kick in.
As Sebastian pointed out, it's relatively easy even for slicing/indexing
functions to check for all-negative suboffsets and handle that case.
All this is probably academic, since no one appears to be using suboffsets
at all. :)
Let's keep the status-quo and make this a doc issue for 2.7.