[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 gvanrossum
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Category: Core (C code)
Group: Python 2.5
Status: Open
Resolution: None
>Priority: 5
Submitted By: Guido van Rossum (gvanrossum)
>Assigned to: Raymond Hettinger (rhettinger)
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: 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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