On Thu, 2022-02-24 at 00:21 +0100, Antonio Cuni wrote: > On Mon, Feb 21, 2022 at 5:18 PM Petr Viktorin <encu...@gmail.com> > wrote: > > Should we care about hacks/optimizations that rely on having the only > > reference (or all references), e.g. mutating a tuple if it has > > refcount > > 1? Immortal objects shouldn't break them (the special case simply > > won't > > apply), but this wording would make them illegal. > > AFAIK CPython uses this internally, but I don't know how > > prevalent/useful it is in third-party code. > > > > FWIW, a real world example of this is numpy.ndarray.resize(..., > refcheck=True): > https://numpy.org/doc/stable/reference/generated/numpy.ndarray.resize.html#numpy.ndarray.resize > https://github.com/numpy/numpy/blob/main/numpy/core/src/multiarray/shape.c#L114 > > When refcheck=True (the default), numpy raises an error if you try to > resize an array inplace whose refcnt > 2 (although I don't understand > why > > 2 and not > 1, and the docs aren't very clear about this). > > That said, relying on the exact value of the refcnt is very bad for > alternative implementations and for HPy, and in particular it is > impossible > to implement ndarray.resize(refcheck=True) correctly on PyPy. So from > this > point of view, a wording which explicitly restricts the "legal" usage > of > the refcnt details would be very welcome.
Yeah, NumPy resizing is a bit of an awkward point, I would be on-board
for just replacing resize for non
NumPy does also have a bit of magic akin to the "string concat" trick
for operations like:
a + b + c
where it will try do magic and use the knowledge that it can
mutate/reuse the temporary array, effectively doing:
tmp = a + b
tmp += c
(which requires some stack walking magic additionally to the refcount!)
Cheers,
Sebastian
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