Expose a dictionary of interned strings in sys module

[35055fa0-fc4d-4752-82be-528e15f2df01]

BPO	34386
Nosy	@rhettinger, @ronaldoussoren, @serhiy-storchaka, @rushter

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GitHub fields: ```python assignee = None closed_at = created_at = labels = ['interpreter-core', 'type-feature', '3.8'] title = 'Expose a dictionary of interned strings in sys module' updated_at = user = 'https://github.com/rushter' ``` bugs.python.org fields: ```python activity = actor = 'serhiy.storchaka' assignee = 'none' closed = True closed_date = closer = 'serhiy.storchaka' components = ['Interpreter Core'] creation = creator = 'rushter' dependencies = [] files = [] hgrepos = [] issue_num = 34386 keywords = [] message_count = 7.0 messages = ['323437', '323440', '323450', '323453', '323455', '323468', '323470'] nosy_count = 4.0 nosy_names = ['rhettinger', 'ronaldoussoren', 'serhiy.storchaka', 'rushter'] pr_nums = [] priority = 'normal' resolution = 'rejected' stage = 'resolved' status = 'closed' superseder = None type = 'enhancement' url = 'https://bugs.python.org/issue34386' versions = ['Python 3.8'] ```

[35055fa0-fc4d-4752-82be-528e15f2df01]

Python provides an ability to intern strings (sys.intern). It would be useful to expose a read-only dictionary of interned strings to the Python users so we can see what kind of strings are interned.

It takes minimal changes since internally it's just a dictionary. Is this worth adding to the sys module?

[ronaldoussoren]

Could you explain why this would be useful?

[rhettinger]

I wouldn't want a user to be able to mutate the dictionary directly (otherwise, non-strings could be added).

[ronaldoussoren]

Another reason for not wanting write access to the sys.intern dictionary is that this dictionary does not own references to its keys and values.

[35055fa0-fc4d-4752-82be-528e15f2df01]

Thank you, I agree. I can't come up with practical use cases other than my curiosity.

Is it possible to somehow expose the dictionary in the debug build of Python? Currently, there is no way to access it from the interpreter even with ctypes.

[ronaldoussoren]

IMHO we shouldn't expose the intern dictionary without there being a clear, and good enough, use case for doing so.

Exposing the dictionary decreases implementation flexibility, and increases requirements on other implementations. One example of the former: at least in theory the interning dictionary could be a set, but we couldn't make that change if the dictionary were exposed in the API.

With current information I'm -1 on exposing the dictionary, and -0 on doing that for debug builds only.

[serhiy-storchaka]

I concur with Ronald. Using a dict instance is an implementation detail. CPython could use a dict, a set, a hashtable implementation from Modules/hashtable.c, or the HAMT implementation from Python/hamt.c. Other Python implementations can have other options.