gh-149083: Convert _initial_missing for pure py reduce to sentinel

[sobolevn]

This is not covered in #149084

Difference is not big, but noticable. Before:

Notice the default parameter repr, which is rather unpretty.

After:

skip news, because there should be no user facing changes.

• Issue: Use PEP 661 (sentinels) in the standard library #149083

[JelleZijlstra]

Maybe ask Hugo how he feels about applying a few more of these in 3.15? It seems fairly low risk when replacing bare object().

[sobolevn]

I agree. It feels like the change should happen together with the addition of sentinel 👍
cc @hugovk

[skirpichev]

It would be nice to fix the C version (it has broken signature now), but it seems that that's not backed yet by the AC. Or did I miss something?

[sobolevn]

Yes, AFAIK sentinel is not yet supported by the AC :(

[skirpichev]

Yes, AFAIK sentinel is not yet supported by the AC :(

Attached patch "works for me":

>>> import inspect, functools
>>> inspect.signature(functools.reduce)
<Signature (function, iterable, /, initial=_initial_missing)>

Details

(Don't forgot to run AC.)

diff --git a/Lib/inspect.py b/Lib/inspect.py
index a96b3dc954e..dc5a6e3be88 100644
--- a/Lib/inspect.py
+++ b/Lib/inspect.py
@@ -2200,7 +2200,8 @@ def wrap_value(s):
             except NameError:
                 raise ValueError
 
-        if isinstance(value, (str, int, float, bytes, bool, type(None))):
+        if isinstance(value, (str, int, float, bytes, bool, type(None),
+                              sentinel)):
             return ast.Constant(value)
         raise ValueError
 
diff --git a/Modules/_functoolsmodule.c b/Modules/_functoolsmodule.c
index 19bdf3d47c2..13da7944b61 100644
--- a/Modules/_functoolsmodule.c
+++ b/Modules/_functoolsmodule.c
@@ -9,6 +9,7 @@
 #include "pycore_tuple.h"         // _PyTuple_ITEMS()
 #include "pycore_weakref.h"       // FT_CLEAR_WEAKREFS()
 
+PyObject *_initial_missing;
 
 #include "clinic/_functoolsmodule.c.h"
 /*[clinic input]
@@ -1066,7 +1067,7 @@ _functools.reduce
     function as func: object
     iterable as seq: object
     /
-    initial as result: object = NULL
+    initial as result: object(c_default="NULL") = _functools._initial_missing
 
 Apply a function of two arguments cumulatively to the items of an iterable, from left to right.
 
@@ -1081,7 +1082,7 @@ calculates ((((1 + 2) + 3) + 4) + 5).
 static PyObject *
 _functools_reduce_impl(PyObject *module, PyObject *func, PyObject *seq,
                        PyObject *result)
-/*[clinic end generated code: output=30d898fe1267c79d input=4ccfb74548ce5170]*/
+/*[clinic end generated code: output=30d898fe1267c79d input=7e5eaeb4f8a7a78d]*/
 {
     PyObject *args, *it;
 
@@ -1982,6 +1983,14 @@ _functools_exec(PyObject *module)
     // lru_list_elem is used only in _lru_cache_wrapper.
     // So we don't expose it in module namespace.
 
+    _initial_missing = PySentinel_New("_initial_missing", "_functools");
+    if (_initial_missing == NULL) {
+        return -1;
+    }
+    if (PyModule_Add(module, "_initial_missing", _initial_missing) < 0) {
+        Py_DECREF(_initial_missing);
+        return -1;
+    }
     return 0;
 }
 

[skirpichev]

This signature line now looks redundant.

[sobolevn]

I change the signature in another PR :)