Jan 15, 2026 · Jan 9, 2026
diff --git a/Lib/test/test_context.py b/Lib/test/test_context.py
            tp.shutdown()
        self.assertEqual(results, list(range(10)))

    def test_context_eq_reentrant_contextvar_set(self):
        var = contextvars.ContextVar("v")
        ctx1 = contextvars.Context()
        ctx2 = contextvars.Context()

        class ReentrantEq:
            def __eq__(self, other):
                ctx1.run(lambda: var.set(object()))
                return True

        ctx1.run(var.set, ReentrantEq())
        ctx2.run(var.set, object())
        ctx1 == ctx2

    def test_context_eq_reentrant_contextvar_set_in_hash(self):
        var = contextvars.ContextVar("v")
        ctx1 = contextvars.Context()
        ctx2 = contextvars.Context()

        class ReentrantHash:
            def __hash__(self):
                ctx1.run(lambda: var.set(object()))
                return 0
            def __eq__(self, other):
                return isinstance(other, ReentrantHash)

        ctx1.run(var.set, ReentrantHash())
        ctx2.run(var.set, ReentrantHash())
        ctx1 == ctx2


 # HAMT Tests

diff --git a/Misc/NEWS.d/next/Core and Builtins/2025-12-17-19-45-10.gh-issue-142829.ICtLXy.rst b/Misc/NEWS.d/next/Core and Builtins/2025-12-17-19-45-10.gh-issue-142829.ICtLXy.rst
 Fix a use-after-free crash in :class:`contextvars.Context` comparison when a
 custom ``__eq__`` method modifies the context via
 :meth:`~contextvars.ContextVar.set`.
diff --git a/Python/hamt.c b/Python/hamt.c
        return 0;
    }

    Py_INCREF(v);
    Py_INCREF(w);

    int res = 1;
    PyHamtIteratorState iter;
    hamt_iter_t iter_res;
    hamt_find_t find_res;
            find_res = hamt_find(w, v_key, &w_val);
            switch (find_res) {
                case F_ERROR:
                    return -1;
                    res = -1;
                    goto done;

                case F_NOT_FOUND:
                    return 0;
                    res = 0;
                    goto done;

                case F_FOUND: {
                    Py_INCREF(v_key);
                    Py_INCREF(v_val);
                    Py_INCREF(w_val);
                    int cmp = PyObject_RichCompareBool(v_val, w_val, Py_EQ);
                    Py_DECREF(v_key);
                    Py_DECREF(v_val);
                    Py_DECREF(w_val);
                    if (cmp < 0) {
                        return -1;
                        res = -1;
                        goto done;
                    }
                    if (cmp == 0) {
                        return 0;
                        res = 0;
                        goto done;
                    }
                }
            }
        }
    } while (iter_res != I_END);

    return 1;
 done:
    Py_DECREF(v);
    Py_DECREF(w);
    return res;
 }

 Py_ssize_t
Original file line number	Diff line number	Diff line change
Expand Up		@@ -370,6 +370,36 @@ def sub(num):
		tp.shutdown()
		self.assertEqual(results, list(range(10)))

		def test_context_eq_reentrant_contextvar_set(self):
		var = contextvars.ContextVar("v")
		ctx1 = contextvars.Context()
		ctx2 = contextvars.Context()

		class ReentrantEq:
		def __eq__(self, other):
		ctx1.run(lambda: var.set(object()))
		return True

		ctx1.run(var.set, ReentrantEq())
		ctx2.run(var.set, object())
		ctx1 == ctx2

		def test_context_eq_reentrant_contextvar_set_in_hash(self):
		var = contextvars.ContextVar("v")
		ctx1 = contextvars.Context()
		ctx2 = contextvars.Context()

		class ReentrantHash:
		def __hash__(self):
		ctx1.run(lambda: var.set(object()))
		return 0
		def __eq__(self, other):
		return isinstance(other, ReentrantHash)

		ctx1.run(var.set, ReentrantHash())
		ctx2.run(var.set, ReentrantHash())
		ctx1 == ctx2


		# HAMT Tests

Expand Down
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,3 @@
		Fix a use-after-free crash in :class:`contextvars.Context` comparison when a
		custom ``__eq__`` method modifies the context via
		:meth:`~contextvars.ContextVar.set`.
Original file line number	Diff line number	Diff line change
Expand Up		@@ -2368,6 +2368,10 @@ _PyHamt_Eq(PyHamtObject v, PyHamtObject w)
		return 0;
		}

		Py_INCREF(v);
		Py_INCREF(w);

		int res = 1;
		PyHamtIteratorState iter;
		hamt_iter_t iter_res;
		hamt_find_t find_res;
Expand All		@@ -2383,25 +2387,38 @@ _PyHamt_Eq(PyHamtObject v, PyHamtObject w)
		find_res = hamt_find(w, v_key, &w_val);
		switch (find_res) {
		case F_ERROR:
		return -1;
		res = -1;
		goto done;

		case F_NOT_FOUND:
		return 0;
		res = 0;
		goto done;

		case F_FOUND: {
		Py_INCREF(v_key);
		Py_INCREF(v_val);
		Py_INCREF(w_val);
		int cmp = PyObject_RichCompareBool(v_val, w_val, Py_EQ);
		Py_DECREF(v_key);
		Py_DECREF(v_val);
		Py_DECREF(w_val);
		if (cmp < 0) {
		return -1;
		res = -1;
		goto done;
		}
		if (cmp == 0) {
		return 0;
		res = 0;
		goto done;
		}
		}
		}
		}
		} while (iter_res != I_END);

		return 1;
		done:
		Py_DECREF(v);
		Py_DECREF(w);
		return res;
		}

		Py_ssize_t
Expand Down