[Patches] [ python-Patches-1433928 ] add on_missing() and default_factory to dict

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Patches item #1433928, was opened at 2006-02-17 19:19
Message generated for change (Comment added) made by rhettinger
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Category: Core (C code)
Group: Python 2.5
Status: Open
>Resolution: Accepted
Priority: 5
Submitted By: Guido van Rossum (gvanrossum)
>Assigned to: Guido van Rossum (gvanrossum)
Summary: add on_missing() and default_factory to dict

Initial Comment:
See the thread starting at
http://mail.python.org/pipermail/python-dev/2006-February/061261.html

This still needs unit tests and docs.

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>Comment By: Raymond Hettinger (rhettinger)
Date: 2006-02-21 08:27

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Neal, the defdict_on_missing check for Py_None is 
necessary because the user can assign None to the factory 
and expect it to have the same effect as NULL.  The other 
checks for NULL all automatically handle the Py_None case 
the same as if the attribute were filled with a real 
factory.

Code nits:
----------

Pre-parse:
  PyEval_CallMethod((PyObject *)mp, "on_missing", "(O)", 
key);
into:
  PyObject_CallMethodObjArgs((PyObject *)mp, "on_missing", 
key, NULL);


The first four lines in defdict_dealloc() can be 
simplified to:
    Py_Clear(dd->default_factory);

Likewise, the first five active lines of defdict_traverse 
shrink to:
    Py_VISIT(dd->default_factory);

The OFF() macro is used only once.  Better to in-line it.


Other
-----

If you don't get a chance to add the docs, just apply the 
patch and I'll write the docs later.

Likewise, I'll update UserDict and DictMixin to keep the 
APIs in-sync.


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Comment By: Neal Norwitz (nnorwitz)
Date: 2006-02-21 02:49

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In test_print() how about os.unlink(tfn)?  Why do you play
games with stdout, can't you do print >>f, ... ?  (If you
don't need stdout, it looks like you don't need sys.)

In defdict_on_missing() why do you check if the factory is
None?  It's the only place None is checked for, everywhere
else, it's just NULL.

defdict_repr should be static.

Type for this line in defdict_init() should be Py_ssize_t,
not int:
int n = PyTuple_GET_SIZE(args);

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Comment By: Guido van Rossum (gvanrossum)
Date: 2006-02-20 20:53

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Here's a completely new version after another round of
python-dev.

- The built-in dict type still defines and calls
on_missing(), but the default on_missing() implementation
just raises KeyError(key).  It no longer has a
default_factory attribute.

- You can subclass dict and override on_missing() to do
whatever you want.

- A useful subclass is collections.defaultdict; it defines
an attribute default_factory and its on_missing()
implementation calls that and inserts the resulting value in
the dict (previous versions of the patch had this semantics
in the built-in dict class, which was frowned upon).

- Now with unit tests.

- No docs yet, though.

Assigning to Raymond Hettinger for review.  Raymond, please
assign it back to me for checkin if you're okay with this
(or for revision if you're not :-).  Because of Google's
lawyers I must check this in myself.

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Comment By: Guido van Rossum (gvanrossum)
Date: 2006-02-19 18:45

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Here's a version that doesn't crash in debug mode.

Neal Norwitz is standing next to me and pointed out that I
was attempting to decref something after tp_free had already
wiped out the object.  D'oh!

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Comment By: Guido van Rossum (gvanrossum)
Date: 2006-02-19 14:00

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Sorry, forgot the upload. Here it is.

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Comment By: Guido van Rossum (gvanrossum)
Date: 2006-02-19 13:57

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Aha. I'll have to try that. In the mean time, here's a new
patch:

- PyDict_GetItem is no longer involved
- added {NULL} to PyMemberDef array
- set ma_default_factory to NULL in both constructors

Still no unit tests or docs.

I have some misgivings about the API -- perhaps this should
be a subclass.


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Comment By: Georg Brandl (birkenfeld)
Date: 2006-02-19 10:39

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Okay. I configured with "--with-pydebug" all the time, and
then comes the segfault.

Without "--with-pydebug", everything seems fine.

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Comment By: Guido van Rossum (gvanrossum)
Date: 2006-02-19 10:28

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Are you sure you did a "make clean"? Because the dictobject
struct lay-out is changed you may have to do that.

If the problem persists, try adding setting
ma_default_factory to NULL explicitly in dict_new (like it's
already done in PyDict_New) and see if that makes a difference.

BTW the change to PyDict_GetItem must be removed -- it's not
a good idea.

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Comment By: Georg Brandl (birkenfeld)
Date: 2006-02-18 05:39

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Observations:
* Doesn't the PyMemberDef need a sentinel?
* Is PyObject_CallObject() faster than PyEval_CallFunction()
with no arguments present?
* The current patch gives me a segfault at interpreter exit
because there is a dict object whose ma_default_factory was
not initialized to NULL.

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