diff --git a/Modules/_datetimemodule.c b/Modules/_datetimemodule.c
index 091da059afd34b..43fd98036991e2 100644
--- a/Modules/_datetimemodule.c
+++ b/Modules/_datetimemodule.c
@@ -1870,17 +1870,20 @@ static PyObject *
 make_freplacement(PyObject *object)
 {
     char freplacement[64];
+    int len;
     if (PyTime_Check(object)) {
-        PyOS_snprintf(freplacement, sizeof(freplacement),
-                      "%06d", TIME_GET_MICROSECOND(object));
+        len = PyOS_snprintf(freplacement, sizeof(freplacement),
+                            "%06d", TIME_GET_MICROSECOND(object));
     }
     else if (PyDateTime_Check(object)) {
-        PyOS_snprintf(freplacement, sizeof(freplacement),
-                      "%06d", DATE_GET_MICROSECOND(object));
+        len = PyOS_snprintf(freplacement, sizeof(freplacement),
+                            "%06d", DATE_GET_MICROSECOND(object));
     }
     else {
-        PyOS_snprintf(freplacement, sizeof(freplacement), "%06d", 0);
+        len = PyOS_snprintf(freplacement, sizeof(freplacement), "%06d", 0);
     }
+    assert(len >= 6);
+    assert(len < sizeof(freplacement));
 
     return PyUnicode_FromString(freplacement);
 }
diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
index e9264f765a08e6..c1f9366e1a634e 100644
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -232,6 +232,18 @@ PyBytes_FromString(const char *str)
     return (PyObject *) op;
 }
 
+static Py_ALWAYS_INLINE int
+_assert_written(int len, Py_ssize_t max_len)
+{
+    assert(len >= 1);
+    assert(len < max_len);
+    return len;
+}
+
+#define SNPRINTF_BYTES(BUFFER, PATTERN, TYP) \
+    _assert_written(PyOS_snprintf(BUFFER, sizeof(BUFFER), \
+                                  PATTERN, va_arg(vargs, TYP)), \
+                    (Py_ssize_t)sizeof(BUFFER))
 
 static char*
 bytes_fromformat(PyBytesWriter *writer, Py_ssize_t writer_pos,
@@ -324,41 +336,37 @@ bytes_fromformat(PyBytesWriter *writer, Py_ssize_t writer_pos,
 
         case 'd':
             if (longflag) {
-                len = sprintf(buffer, "%ld", va_arg(vargs, long));
+                len = SNPRINTF_BYTES(buffer, "%ld", long);
             }
             else if (size_tflag) {
-                len = sprintf(buffer, "%zd", va_arg(vargs, Py_ssize_t));
+                len = SNPRINTF_BYTES(buffer, "%zd", Py_ssize_t);
             }
             else {
-                len = sprintf(buffer, "%d", va_arg(vargs, int));
+                len = SNPRINTF_BYTES(buffer, "%d", int);
             }
-            assert(1 <= len && len < (Py_ssize_t)sizeof(buffer));
             WRITE_BYTES(buffer, len);
             break;
 
         case 'u':
             if (longflag) {
-                len = sprintf(buffer, "%lu", va_arg(vargs, unsigned long));
+                len = SNPRINTF_BYTES(buffer, "%lu", unsigned long);
             }
             else if (size_tflag) {
-                len = sprintf(buffer, "%zu", va_arg(vargs, size_t));
+                len = SNPRINTF_BYTES(buffer, "%zu", size_t);
             }
             else {
-                len = sprintf(buffer, "%u", va_arg(vargs, unsigned int));
+                len = SNPRINTF_BYTES(buffer, "%u", unsigned int);
             }
-            assert(1 <= len && len < (Py_ssize_t)sizeof(buffer));
             WRITE_BYTES(buffer, len);
             break;
 
         case 'i':
-            len = sprintf(buffer, "%i", va_arg(vargs, int));
-            assert(1 <= len && len < (Py_ssize_t)sizeof(buffer));
+            len = SNPRINTF_BYTES(buffer, "%i", int);
             WRITE_BYTES(buffer, len);
             break;
 
         case 'x':
-            len = sprintf(buffer, "%x", va_arg(vargs, int));
-            assert(1 <= len && len < (Py_ssize_t)sizeof(buffer));
+            len = SNPRINTF_BYTES(buffer, "%x", int);
             WRITE_BYTES(buffer, len);
             break;
 
@@ -382,8 +390,7 @@ bytes_fromformat(PyBytesWriter *writer, Py_ssize_t writer_pos,
         }
 
         case 'p':
-            len = sprintf(buffer, "%p", va_arg(vargs, void*));
-            assert(1 <= len && len < (Py_ssize_t)sizeof(buffer));
+            len = SNPRINTF_BYTES(buffer, "%p", void*);
             /* %p is ill-defined:  ensure leading 0x. */
             if (buffer[1] == 'X') {
                 buffer[1] = 'x';
@@ -415,6 +422,7 @@ bytes_fromformat(PyBytesWriter *writer, Py_ssize_t writer_pos,
     return s;
 
 #undef WRITE_BYTES
+#undef SNPRINTF_BYTES
 }