C API: Add PyDict_GetItemRef() function

The PyDict C API has a bad history. PyDict_GetItem() ignores all exception: error on hash(), error on "key == key2", KeyboardInterrupt, etc. PyDict_GetItemWithError() was added to fix this design. Moreover, Python 3.9 and older allowed to call PyDict_GetItem() with the GIL released.

PyDict_GetItem() returns a borrowed reference which is usually safe since the dictionary still contains a strong reference to the request value. But in general, borrowed references are error prone and can likely lead to complex race conditions causing crashes:

"Functions must not return borrowed references" says https://devguide.python.org/developer-workflow/c-api/index.html
Unclear lifetimes of borrowed references capi-workgroup/problems#5

While PyDict_GetItem() calls can be quite easily replaced with PyObject_GetItem() which has a better API (return a new stong reference), developers usually prefer to still use the specialized PyDict API for best performance: avoid the minor overhead of type dispatching, Py_TYPE(obj)->tp_as_mapping->mp_subscript.

I propose adding PyDict_GetItemRef() and PyDict_GetItemStringRef() functions to the limited C API (version 3.13): replacements for PyDict_GetItem(), PyDict_GetItemWithError() and PyDict_GetItemString().

API:

int PyDict_GetItemRef(PyObject *mp, PyObject *key, PyObject **pvalue);
int PyDict_GetItemStringRef(PyObject *mp, const char *key, PyObject **pvalue);

PyDict_GetItemWithError() has another API issue: when it returns NULL, it can mean two things. It returns NULL if the key is missing, but it also returns NULL on error. The caller has to check PyErr_Occurred() to distinguish the two cases (to write correct code). See capi-workgroup/problems#1 Proposed API avoids this by returning an int: return -1 on error, or return 0 otherwise (present or missing key). Checking PyErr_Occurred() is no longer needed.

By the way, the public C API has no PyDict_GetItemStringWithError() function: using PyDict_GetItemWithError() with a char* key is not convenient. The _PyDict_GetItemStringWithError() function exists but it's a private C API.

Linked PRs

Example of PyDict_GetItemRef() usage of the function, replacing PyDict_GetItemWithError():

@@ -5146,13 +5191,12 @@ dictitems_contains(_PyDictViewObject *dv, PyObject *obj)
         return 0;
     key = PyTuple_GET_ITEM(obj, 0);
     value = PyTuple_GET_ITEM(obj, 1);
-    found = PyDict_GetItemWithError((PyObject *)dv->dv_dict, key);
+    if (PyDict_GetItemRef((PyObject *)dv->dv_dict, key, &found) < 0) {
+        return -1;
+    }
     if (found == NULL) {
-        if (PyErr_Occurred())
-            return -1;
         return 0;
     }
-    Py_INCREF(found);
     result = PyObject_RichCompareBool(found, value, Py_EQ);
     Py_DECREF(found);
     return result;

PyErr_Occurred() and Py_INCREF() calls can be removed.

pythongh-106004: Add PyDict_GetItemRef() function

See also my previous attempt in 2020: issue #86460 "Add new C functions with more regular reference counting like PyTuple_GetItemRef()".

pythongh-106004: Add PyDict_GetItemRef() function

I'm in favor of this because I don't think we should have public APIs that (a) require a value check + PyErr_Occurred() call pattern - a frequent source of lurking bugs - or (b) return borrowed references. Yes I know we already have them, that's missing the point. The point is that with these in place, we can promote their use over the others because these are better in all respects.

One possible long term future would have us deprecate the messy borrowing APIs. Having these already in place sooner rather than later smooths that potential transition.

There is no foreseeable future in which we'd go the other way towards all-borrowing-only APIs.

There is no foreseeable future in which we'd go the other way towards all-borrowing-only APIs.

FYI I'm tracking the list of C API functions returning borrowed references at: https://pythoncapi.readthedocs.io/bad_api.html#functions

I'm trying to provide a replacement for each function, and if possible an efficient replacement. For example, PyDict_GetItem() can be replaced with PyObject_GetItem(), but it's less efficient (that's what I explained in my first message).

pythongh-106004: Add PyDict_GetItemRef() function

The only problem is that functions with so similar names have completely different interface. It is pretty confusing. Would not be better to name it PyDict_LookupItem or like? It may be worth to add also PyMapping_LookupItem for convenience. BTW, I think it is time to make _PyObject_LookupAttr public (if we agree about the name).

I replied on the PR, since the discussion is ongoing there.

pythongh-106004: Add PyDict_GetItemRef() function

gh-106004: Add PyDict_GetItemRef() function (#106005)

Merge branch 'main' into superfix

pythongh-106004: PyDict_GetItemRef() should only be in the limited AP…

Tests will be added in #107179.

gh-106004: PyDict_GetItemRef() should only be in the limited API 3.13 (…

pythongh-106004: PyDict_GetItemRef() should only be in the limited AP…

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C API: Add PyDict_GetItemRef() function #106004

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C API: Add PyDict_GetItemRef() function #106004

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vstinner commented on Jun 23, 2023

vstinner commented on Jun 23, 2023

gpshead commented on Jun 29, 2023

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