Issue 30486: Allow setting cell value

There are use cases for setting a cell value.  One such use case is for (un)pickling recursive closures (see heroic workaround here: https://github.com/cloudpipe/cloudpickle/pull/90/files#diff-d2a3618afedd4e124c532151eedbae09R74 ).  Other use cases may include tinkering around and general education value.

There also doesn't seem to be, AFAICS, any counter-indication to being able to do so.  It's already possible in C using PyCell_Set(), which is a public API.  It just lacks an API in Python land.  For example `cell_contents` could become a read/write property...

This idea makes sense to me. I'll see if I can put together the code for it.

+1  I don't see any reason this can't be writeable.

+1 from me, as this also has potential value in testing use cases and in dealing with some of the consequences of the zero-arg super() design (i.e. it will make the injected __class__ cell writable)

A possible counter-indication would be if setting a cell could cause a crash, as opposed to a mysterious exception.  Since 3.x already allows writing any object to a cell from python code,
 
def outer():
    cell = None
    def inner(ob):
        nonlocal cell
        cell = ob  # rebinds <hidden-cell>.cell_contents
    return inner

set_cell = outer()
print(set_cell.__closure__[0].cell_contents)  # None
set_cell('something')
print(set_cell.__closure__[0].cell_contents)  # 'something'

making "cell.cell_contents = 'something'" legal should not enable more crashes.

I think that "function.__closure__[i].cell_contents = object" is perhaps not a good idea for production code, but I think it falls within the realm of 'consenting adults code' for the other uses suggested above.  How should it be documented?

There's already a documentation change to the PR, you can leave your comments there.

New changeset 64505a1f6c0af4574e17e823b27ffe24eca44df5 by Serhiy Storchaka (Lisa Roach) in branch 'master':
bpo-30486: Allow setting cell value (#1840)
https://github.com/python/cpython/commit/64505a1f6c0af4574e17e823b27ffe24eca44df5

Thank you for your contribution Lisa!

Thank you for you guidance, Serhiy!

On Thu, Jun 8, 2017 at 4:45 AM, Serhiy Storchaka <report@bugs.python.org>
wrote:

>
> Serhiy Storchaka added the comment:
>
> Thank you for your contribution Lisa!
>
> ----------
> resolution:  -> fixed
> stage: patch review -> resolved
> status: open -> closed
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <http://bugs.python.org/issue30486>
> _______________________________________
>

New changeset 0ad05c32cc41d4c21bfd78b9ffead519ead475a2 by Victor Stinner in branch 'master':
bpo-30486: Make cell_set_contents() symbol private (#3668)
https://github.com/python/cpython/commit/0ad05c32cc41d4c21bfd78b9ffead519ead475a2

stage: patch review -> resolved

messages: + msg295057
stage: needs patch -> patch review