Add an empty frozendict singleton

[vstinner]

I propose adding an empty frozendict singleton.

For example, it would be returned on the following operations:

• frozendict()
• frozendict({})
• frozendict() | frozendict()
• frozendict() | {}
• frozendict.fromkeys([])

Linked PRs

• gh-153060: Add an empty frozendict singleton #153061

[vstinner]

An empty frozendict singleton should reduce the memory consumption, since all empty frozendict objects reuse the same unique object (which is 72 bytes on x86-64).

It should also reduce refcount contention in Free Threading, since the singleton is created immortal.

[jonathandung]

Seeing this issue, I wonder why there was never a frozenset singleton:

>>> id(frozenset())
1586521037216
>>> id(frozenset())
1586521034080

// Include/object.h

#define Py_CONSTANT_NONE 0
#define Py_CONSTANT_FALSE 1
#define Py_CONSTANT_TRUE 2
#define Py_CONSTANT_ELLIPSIS 3
#define Py_CONSTANT_NOT_IMPLEMENTED 4
#define Py_CONSTANT_ZERO 5
#define Py_CONSTANT_ONE 6
#define Py_CONSTANT_EMPTY_STR 7
#define Py_CONSTANT_EMPTY_BYTES 8
#define Py_CONSTANT_EMPTY_TUPLE 9

[jonathandung]

Or should I open a discussion thread for this?

[vstinner]

Seeing this issue, I wonder why there was never a frozenset singleton

Python had a empty frozenset singleton until Python 3.9. #21085 removed it in Python 3.10.

I would prefer to stick to frozendict in this issue. If you consider that frozenset deserves an empty singleton again, and you measured some benefits (with an implementation if possible), you can open a different issue for it.