Backport PR #40525 on branch 1.2.x (BUG: to_json failing on PyPy)

doc/source/whatsnew/v1.2.4.rst

@@ -16,6 +16,7 @@ Fixed regressions
 ~~~~~~~~~~~~~~~~~
 
 - Fixed regression in :meth:`DataFrame.sum` when ``min_count`` greater than the :class:`DataFrame` shape was passed resulted in a ``ValueError`` (:issue:`39738`)
+- Fixed regression in :meth:`DataFrame.to_json` raising ``AttributeError`` when run on PyPy (:issue:`39837`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/_libs/src/ujson/python/objToJSON.c

@@ -272,18 +272,6 @@ static PyObject *get_sub_attr(PyObject *obj, char *attr, char *subAttr) {
     return ret;
 }
 
-static int is_simple_frame(PyObject *obj) {
-    PyObject *check = get_sub_attr(obj, "_mgr", "is_mixed_type");
-    int ret = (check == Py_False);
-
-    if (!check) {
-        return 0;
-    }
-
-    Py_DECREF(check);
-    return ret;
-}
-
 static Py_ssize_t get_attr_length(PyObject *obj, char *attr) {
     PyObject *tmp = PyObject_GetAttrString(obj, attr);
     Py_ssize_t ret;
@@ -301,6 +289,17 @@ static Py_ssize_t get_attr_length(PyObject *obj, char *attr) {
     return ret;
 }
 
+static int is_simple_frame(PyObject *obj) {
+    PyObject *mgr = PyObject_GetAttrString(obj, "_mgr");
+    if (!mgr) {
+        return 0;
+    }
+    int ret = (get_attr_length(mgr, "blocks") <= 1);
+
+    Py_DECREF(mgr);
+    return ret;
+}
+
 static npy_int64 get_long_attr(PyObject *o, const char *attr) {
     npy_int64 long_val;
     PyObject *value = PyObject_GetAttrString(o, attr);