3.12 setrecursionlimit is ignored in connection with @functools.cache

@dimpase

Bug report

A minimal example:

#  fib.py 
import sys
sys.setrecursionlimit(2000)

from functools import cache

@cache
def fib(n):
    if n<1: return 0
    if n==1: return 1
    return fib(n-1) + fib(n-2)

print(fib(500))

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Edit: This (simpler) example is from @dimpase .

Linked PRs

also tested on Linux (with the call to sys.getwindowsversion() removed).

As well, there is no discrepancy with 3.11 vs 3.12 if @cache is removed. So the working hypothesis is that @cache is to blame for the regression.

The current master is bad too.
Bisecting currently, last year things were still OK.

In my testing I modified the lines

    sys.set_int_max_str_digits(5000)
    print(sys.version_info, sys.getwindowsversion())

I removed the 1st one (it's very recent), and only left

    print(sys.version_info)

I found the 1st bad commit, 7644935, by bisecting:

76449350b3467b85bcb565f9e2bf945bd150a66e is the first bad commit
commit 76449350b3467b85bcb565f9e2bf945bd150a66e
Author: Mark Shannon <mark@hotpy.org>
Date:   Wed Oct 5 01:34:03 2022 +0100

    GH-91079: Decouple C stack overflow checks from Python recursion checks. (GH-96510)

 Include/cpython/pystate.h                          | 16 ++++-
 Include/internal/pycore_ceval.h                    | 21 +++---
 Include/internal/pycore_runtime_init.h             |  2 +-
 Lib/test/support/__init__.py                       |  5 +-
 Lib/test/test_ast.py                               |  6 +-
 Lib/test/test_call.py                              | 38 ++++++++++
 Lib/test/test_collections.py                       |  2 +-
 Lib/test/test_compile.py                           |  3 +-
 Lib/test/test_dynamic.py                           |  8 +--
 Lib/test/test_exceptions.py                        |  8 +--
 Lib/test/test_isinstance.py                        | 12 ++--
 Lib/test/test_marshal.py                           |  3 +-
 .../2022-09-05-09-56-32.gh-issue-91079.H4-DdU.rst  |  3 +
 Modules/_testinternalcapi.c                        |  4 +-
 Parser/asdl_c.py                                   |  9 +--
 Python/Python-ast.c                                |  9 +--
 Python/ast.c                                       |  9 +--
 Python/ast_opt.c                                   |  9 +--
 Python/ceval.c                                     | 81 +++++++++++++++-------
 Python/pystate.c                                   |  5 +-
 Python/symtable.c                                  |  9 +--
 Python/sysmodule.c                                 |  2 +-
 22 files changed, 165 insertions(+), 99 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Core and Builtins/2022-09-05-09-56-32.gh-issue-91079.H4-DdU.rst

@markshannon

@markshannon - greetings from Oxford, by the way :-)

Yes. I was suggesting exposing an API for the C recursion limit, but not receiving active feedback.

See the discussion here: #107263

@PeterLuschny

@PeterLuschny - basic Fibonacci

#  fib.py 
import sys
sys.setrecursionlimit(2000)

from functools import cache

@cache
def fib(n):
    if n<1: return 0
    if n==1: return 1
    return fib(n-1) + fib(n-2)

print(fib(500))

shows the bug just as well

it's not "probably in connection with @cache", it's definitely

The bug is triggered by the use of the C implementation of _lru_cache_wrapper, in Modules/_functoolsmodule.c.
To see this, disable it (then Python code will be used) by applying

--- a/Lib/functools.py
+++ b/Lib/functools.py
@@ -641,7 +641,7 @@ def cache_clear():
     return wrapper
 
 try:
-    from _functools import _lru_cache_wrapper
+    from _functools import no_no_lru_cache_wrapper
 except ImportError:
     pass

and see ./python <fib.py computing the answer, instead of erroring out.

In a way, it's to be expected, as C code isn't governed by setrecursionlimit any more in 3.12+.

@arhadthedev

@arhadthedev This is not related to performance. It's a new hard-coded limit not adjustable.

Add test_long_kernel

pythonGH-112215: Increase C recursion limit for non debug builds (pyt…

~~What's the plan for 3.13? It looks like it still doesn't behave as expected~~

I cannot reproduce error from #127346 on my macos, so it's probably a Windows issue?

beware that C stack size issues are typically local platform & build config dependent as to what numbers you need to use to bump into a given issue.

Another user reported this as a bug: #127346

Also there is the question on StackOverflow: https://stackoverflow.com/questions/78389885/why-lru-cache-doesnt-work-on-desktops-but-only-works-in-the-online-python-int

For the particular case of cache() and lru_cache(), we may need to provide an easy way to get to the pure python version that works as expected. Perhaps something like from functools import py_lru_cache as lru_cache. Without such an option, users would need to code their own caching decorator to be able to run dynamic programming like that found in the StackOverflow question. That is what the user had to do in #127346 . I'm pretty sure that is not what we want.

I think the solution to this is to implement the wrapper function in Python, but implement the cache management in C.
That will fix the recursion issue, with little or no performance impact.

The wrapper function would look something like this:

from _functools import make_key, cache_get, cache_set, SENTINEL

def wrapper(*args, **kwds):
    key = make_key(args, kwds, typed)
    result = cache_get(cache, key)
    if result is not SENTINEL:
        return result
    result = user_function(*args, **kwds)
    cache_set(cache, key, result)
    return result

This is fixed in 3.14.

The fix is backported to 3.13 & 3.12.

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3.12 setrecursionlimit is ignored in connection with `@functools.cache` #112215

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A minimal example:

CPython versions tested on:

Operating systems tested on:

Linked PRs

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3.12 setrecursionlimit is ignored in connection with @functools.cache #112215

Description

Bug report

A minimal example:

CPython versions tested on:

Operating systems tested on:

Linked PRs

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dimpase commented on Nov 17, 2023

dimpase commented on Nov 17, 2023

dimpase commented on Nov 17, 2023

sunmy2019 commented on Nov 18, 2023

dimpase commented on Nov 18, 2023

dimpase commented on Nov 18, 2023

dimpase commented on Nov 18, 2023

sunmy2019 commented on Nov 20, 2023

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Eclips4 commented on Nov 27, 2024

gpshead commented on Nov 27, 2024

rhettinger commented on Jan 12, 2025

markshannon commented on Feb 12, 2025

markshannon commented on May 6, 2025

encukou commented on Nov 20, 2025

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3.12 setrecursionlimit is ignored in connection with `@functools.cache` #112215