Accessing the contents of a 'cell' object from Python

[paul cannon]

Having poked around a little bit, I found there doesn't appear to be any
way to get at the contents of a cell object from Python. It's not the
sort of thing that one needs to be doing very frequently, but I've run
into a few situations recently where it would be really useful from a
debugging standpoint.

You can get at a cell object containing a given value by making a quick
closure and looking at the func_closure attribute:

  (lambda x: lambda: x)(some_value).func_closure[0]

but there's not anything we can easily do with that object to find out
what it's pointing at. The str() representation helpfully tells us the 
id of the contained value, but I'm not aware of a way to get at an
object given its id [1].

So I thought it might be useful to add such a method to the cell object
type. Attached is a sample patch that does so (against python2.4). Does
this look like the sort of thing that would be useful to anyone else?

How does one go about getting small changes like this into Python
anyway?

-- 
paul

[1] Yes, the id is the object's pointer in cPython, but even I don't
think it would be a good idea to allow accessing an object via the id,
even if it's possible. :)
diff -Naur python2.4-2.4dfsg/Objects/cellobject.c 
python2.4-2.4dfsg-pik2/Objects/cellobject.c
--- python2.4-2.4dfsg/Objects/cellobject.c  2001-08-29 17:51:00.0 
-0600
+++ python2.4-2.4dfsg-pik2/Objects/cellobject.c 2005-03-15 14:21:02.389725784 
-0700
@@ -86,6 +86,13 @@
return 0;
 }
 
+static PyMethodDef cell_methods[] = {
+   {"value", (PyCFunction)PyCell_Get, METH_NOARGS,
+ "Return the value stored by the cell"
+   },
+   {NULL} /* sentinel */
+};
+
 PyTypeObject PyCell_Type = {
PyObject_HEAD_INIT(&PyType_Type)
0,
@@ -111,4 +118,9 @@
0,  /* tp_doc */
(traverseproc)cell_traverse,/* tp_traverse */
(inquiry)cell_clear,/* tp_clear */
+   0,  /* tp_richcompare */
+   0,  /* tp_weaklistoffset */
+   0,  /* tp_iter */
+   0,  /* tp_iternext */
+   cell_methods,   /* tp_methods */
 };
-- 
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Re: Accessing the contents of a 'cell' object from Python

[paul cannon]

On Tue, Mar 15, 2005 at 03:08:19PM -0700, paul cannon wrote:
> Having poked around a little bit, I found there doesn't appear to be any
> way to get at the contents of a cell object from Python. It's not the
> sort of thing that one needs to be doing very frequently, but I've run
> into a few situations recently where it would be really useful from a
> debugging standpoint.

Okay, I did come up with one solution- create a new function that just
returns a value from its own closure, and manufacture its closure from
the cell you already have.

So..

  import new
  def get_cell_value(cell):
  return new.function(
  (lambda x: lambda: x)(0).func_code, {}, None, None, (cell,)
  )()

It could be optimized a bit by precalculating the code object.

I do still think the earlier solution (giving the cell objects a method)
is cleaner and better long-term, but this will do.

-- 
paul
-- 
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