Interned strings are immortal, despite what the documentation says

[encukou]

Bug report

Bug description:

The sys.intern documentation explicitly says:

Interned strings are not immortal; you must keep a reference to the return value of intern() around to benefit from it.

However, they were made immortal in #19474 (Implement Immortal Objects -- implementation of PEP 683), without a documentation update. The 3.12 What's New also doesn't mention the change. [edit: it's now clear that the PR author (@eduardo-elizondo) intended it but the reviewer (@markshannon) did not]
PEP-683 itself only mentions the change as a possible optimization.

Meanwhile, pathlib interns every path segment it processes, presumably depending on the documented behaviour. In CPython's test suite, that causes names of temporary directories to leak (stealthily, since interned strings are excluded from the total refcount).
[edit: Worse, type_setattro, PyObject_SetAttr or PyDict_SetItemString immortalize keys, so strings used for key/attribute names this way are not reclaimed until interpreter shutdown.]
On the other hand, free-threading (PEP-703) seems to rely [edit: relies] on these being immortal.

What's the correct behaviour? [edit: specifically, in 3.12 and non-free-threading 3.13, should this change be reverted or documented?]

@eduardo-elizondo @ericsnowcurrently

Linked PRs

• gh-113993: Allow interned strings to be mortal, and fix related issues #120520
• [3.13] gh-113993: Allow interned strings to be mortal, and fix related issues (GH-120520) #120945
• gh-113993: For string interning, do not rely on (or assert) _Py_IsImmortal #121358
• gh-113993: Don't immortalize in PyUnicode_InternInPlace; keep immortalizing in other API #121364
• [3.13] gh-113993: For string interning, do not rely on (or assert) _Py_IsImmortal (GH-121358) #121851
• [3.13] gh-113993: Don't immortalize in PyUnicode_InternInPlace; keep immortalizing in other API (GH-121364) #121854
• [3.12] gh-113993: Make interned strings mortal (GH-120520, GH-121364, GH-121903, GH-122303) #123065
• [3.12] gh-113993: For string interning, do not rely on (or assert) _Py_IsImmortal (GH-121358) (GH-121851) #124938
• gh-113993: InternalDocs: Add String Interning to README #127250

Related fixes:

• gh-121863: Immortalize names in code objects to avoid crash #121903
• gh-122291: Intern latin-1 one-byte strings at startup #122303
• gh-122420: Fix accounting for immortal interned strings in refleak.py #122421

[nascheme]

I would suggest that using "immortal" here could be confusing. The flagged checked by _Py_IsImmortal() is low-level implementation detail and should not be something that the library documentation discusses. Strings that you call intern() on may or may not have this flag set. Different Python runtimes might implement intern() some other way. The behavior that matters is that the string gets stored in an interned strings table and that future calls to intern() with the same string content will return an object with the same id(), even if the user program doesn't keep a reference to the string.

[encukou]

The behavior that matters is that the string gets stored in an interned strings table and that future calls to intern() with the same string content will return an object with the same id(), even if the user program doesn't keep a reference to the string.

No. That is not the behaviour on Python 3.11.
I get:

$ python3.11
Python 3.11.7 (main, Dec 18 2023, 00:00:00) [GCC 13.2.1 20231205 (Red Hat 13.2.1-6)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> id1 = id(sys.intern('some string'))
>>> other = sys.intern('other string')
>>> id2 = id(sys.intern('some string'))
>>> id1 == id2
False

IMO, the guarantee is that if two strings are interned, they can be compared by identity.
CPython uses this for a fast path in string comparison; so some users call intern on strings that are likely to be compared to one another -- even ones that srent expected to live until interpreter shutdown. We can say that's optmizing for a specific implementation, but it's common -- even pathlib and the compiler (e.g. eval) do it.

The new behaviour can leak strings that are not likely to be used again.

[erlend-aasland]

$ cat test.py
import sys
id1 = id(sys.intern('some string'))
other = sys.intern('other string')
id2 = id(sys.intern('some string'))
assert id1 == id2
$ python3.8 ./test.py
$ python3.9 ./test.py
$ python3.10 ./test.py
$ python3.11 ./test.py
$ python3.12 ./test.py
$ python3.13 ./test.py

No assertions triggered.

[encukou]

The the new string is allowed to reuse the old one's memory; I guess this is build-specific. What platform are you on?
Instead of other = sys.intern('other string'), try putting in gc.collect and something more intensive, like import asyncio?

[erlend-aasland]

I'm on macOS using the official build. Strange thing: I can reproduce the behaviour you're seeing in the REPL.

[erlend-aasland]

Instead of other = sys.intern('other string'), try putting in gc.collect and something more intensive, like import asyncio?

Nope, cannot reproduce using $ python3.11 ./test.py as above.

[encukou]

Ah! Right, literal constants are part the code object, which does normally stay around. You'll need something that's not folded to a constant, like:

$ cat test.py 
import sys
def intern_and_report(string):
    string = sys.intern(string)
    the_id = id(string)
    print(f'{string!r} at {id(string)}: refcount {sys.getrefcount(string):x}')
    return the_id

id1 = intern_and_report('a' + __name__)
str_b = sys.intern('b' + __name__)
id2 = intern_and_report('a' + __name__)
assert id1 == id2

$ python3.10 ./test.py
'a__main__' at 139775546163056: refcount 3
'a__main__' at 139775546294640: refcount 3
Traceback (most recent call last):
  File "/tmp/rttair/./test.py", line 11, in <module>
    assert id1 == id2
AssertionError

$ python3.12 ./test.py
'a__main__' at 139964910181616: refcount ffffffff
'a__main__' at 139964910181616: refcount ffffffff

[erlend-aasland]

Yes; thanks!

[erlend-aasland]

See also #113601

[eduardo-elizondo]

@encukou just getting to this! You're right, as of now, all interned strings should be immortal, so the sys.intern docs should probably be updated to update that they are indeed immortal and the fact that we don't need to keep a reference to benefit from it anymore.

As for the leaks, as @erlend-aasland already called out, we have a PR to correctly clean them up in #113601!

[nascheme]

The behavior that matters is that the string gets stored in an interned strings table and that future calls to intern() with the same string content will return an object with the same id(), even if the user program doesn't keep a reference to the string.

No. That is not the behaviour on Python 3.11.

Sorry, I was not very clear. I was describing the behaviour we have with Python 3.12 and we had with older versions of Python (version 2.2 and before, it seems). It looks like this commit changed it: 45ec02a.

Since 3.12 changed this, definitely the documentation should be updated. Further, I think we should have had a discussion and explicitly decide if this is desirable behaviour, since it reverts the effect of 45ec02a. Maybe there was one and I missed it? Perhaps free-threading doesn't require that every interned string become immortal but only some of them (e.g. symbols used by code objects etc). That might preserve free-threaded performance (e.g. interned strings shared between threads) while avoid the "leak" like encountered by pathlib (e.g. interned strings only used for a short time and then discarded).

[encukou]

@eduardo-elizondo

as of now, all interned strings should be immortal

To be clear, I think this is bad. Some interned strings are temporary, and they now leak. They should be cleaned up sooner that at interpreter shutdown.