[Python-Dev] Add a frozendict builtin type

[Victor Stinner]

>>  * frozendict values must be immutable, as dict keys
>
> Why?  That may be useful, but an immutable dict whose values
> might mutate is also useful; by forcing that choice, it starts
> to feel too specialized for a builtin.

Hum, I realized that calling hash(my_frozendict) on a frozendict
instance is enough to check if a frozendict only contains immutable
objects. And it is also possible to check manually that values are
immutable *before* creating the frozendict.

I also prefer to not check for immutability because it does simplify
the code :-)

$ diffstat frozendict-3.patch
 Include/dictobject.h   |    9 +
 Lib/collections/abc.py |    1
 Lib/test/test_dict.py  |   59 +++++++++++
 Objects/dictobject.c   |  256 ++++++++++++++++++++++++++++++++++++++++++-------
 Objects/object.c       |    3
 Python/bltinmodule.c   |    1
 6 files changed, 295 insertions(+), 34 deletions(-)

The patch is quite small to add a new builtin type. That's because
most of the code is shared with the builtin dict type. (But the patch
doesn't include the documentation, it didn't write it yet.)

Victor
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