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PEP 574: update #883

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ncoghlan merged 2 commits into from
Jan 27, 2019
Merged

PEP 574: update #883

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PEP 574: add bit of trivia about the undocumented "inst_persistent_id…
pitrou Jan 23, 2019
f61c400
PEP 574: make it a requirement that buffers are contiguous
pitrou Jan 23, 2019
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21 changes: 18 additions & 3 deletions pep-0574.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -212,10 +212,20 @@ PickleBuffer objects:
Release the ``PickleBuffer`` instance's underlying buffer.


Buffer requirements
-------------------

``PickleBuffer`` can wrap any kind of buffer, including non-contiguous
buffers. It's up to consumers to decide how best to handle different kinds
of buffers (for example, some consumers may find it acceptable to make a
contiguous copy of non-contiguous buffers).
buffers. However, it is required that ``__reduce__`` only returns a
contiguous ``PickleBuffer`` (*contiguity* here is meant in the PEP 3118
sense: either C-ordered or Fortran-ordered). Non-contiguous buffers
will raise an error when pickled.

This restriction is primarily an ease-of-implementation issue for the
``pickle`` module but also other consumers of out-of-band buffers.
The simplest solution on the provider side is to return a contiguous
copy of a non-contiguous buffer; a sophisticated provider, though, may
decide instead to return a sequence of contiguous sub-buffers.


Consumer API
Expand Down Expand Up @@ -420,6 +430,11 @@ This mechanism has two drawbacks:
such as ints and strings) triggers a call to the user's ``persistent_id()``
method, leading to a possible performance drop compared to nominal.

(the Python 2 ``cPickle`` module supported an undocumented
``inst_persistent_id()`` hook that was only called on non-built-in types;
it was added in 1997 in order to alleviate the performance issue of
calling ``persistent_id``, presumably at ZODB's request)

Passing a sequence of buffers in ``buffer_callback``
----------------------------------------------------

Expand Down