Disable time collection when display is disabled.

TODO: performance testing - this increases the conditional load on every
BaseException instantiation with that interp->config.field && field[0] check.

If needed, we could cache the "collect or not" bool in a static global as it is
fair to say this is a process wide setting rather than per interpreter, but ugh.

Author: gpshead

Doc/c-api/init_config.rst

@@ -493,6 +493,10 @@ Configuration Options
      - :c:member:`tracemalloc <PyConfig.tracemalloc>`
      - ``int``
      - Read-only
+   * - ``"traceback_timestamps"``
+     - :c:member:`traceback_timestamps <PyConfig.traceback_timestamps>`
+     - ``str``
+     - Read-only
    * - ``"use_environment"``
      - :c:member:`use_environment <PyConfig.use_environment>`
      - ``bool``
@@ -1879,6 +1883,25 @@ PyConfig
 
       Default: ``-1`` in Python mode, ``0`` in isolated mode.
 
+   .. c:member:: wchar_t* traceback_timestamps
+
+      Format of timestamps shown in tracebacks.
+
+      If not ``NULL`` or an empty string, timestamps of exceptions are collected
+      and will be displayed in the configured format. Acceptable values are:
+
+      * ``"us"``: Display timestamps in microseconds
+      * ``"ns"``: Display timestamps in nanoseconds
+      * ``"iso"``: Display timestamps in ISO-8601 format
+      * ``""``: Collection and display is disabled.
+
+      Set by the :option:`-X traceback_timestamps=FORMAT <-X>` command line
+      option or the :envvar:`PYTHON_TRACEBACK_TIMESTAMPS` environment variable.
+
+      Default: ``NULL`` (timestamps disabled).
+
+      .. versionadded:: next
+
    .. c:member:: int perf_profiling
 
       Enable the Linux ``perf`` profiler support?

Lib/test/test_interpreters/utils.py

@@ -458,8 +458,10 @@ def make_module(self, name, pathentry=None, text=None):
 
     @support.requires_subprocess()
     def run_python(self, *argv):
+        # Make assertions of specific traceback output simpler.
+        arguments = ["-X", "traceback_timestamps=0", *argv]
         proc = subprocess.run(
-            [sys.executable, *argv],
+            [sys.executable, *arguments],
             capture_output=True,
             text=True,
         )

Lib/test/test_traceback_timestamps.py

@@ -143,7 +143,7 @@ def test_traceback_timestamps_invalid_flag(self):
         """Test that invalid flag values cause an error"""
         result = script_helper.assert_python_failure("-X", "traceback_timestamps=invalid", self.flags_script_path)
         stderr = result.err.decode()
-        self.assertIn("Invalid value for -X traceback_timestamps option", stderr)
+        self.assertIn("Invalid -X traceback_timestamps=value option", stderr)
 
 
 if __name__ == "__main__":

Objects/exceptions.c

@@ -41,6 +41,16 @@ get_exc_state(void)
 }
 
 
+static bool
+should_collect_traceback_timestamps(void)
+{
+    /* Unset or empty means disabled. */
+    wchar_t *traceback_timestamps = (
+        _PyInterpreterState_GET()->config.traceback_timestamps);
+    return traceback_timestamps && traceback_timestamps[0] != '\0';
+}
+
+
 /* NOTE: If the exception class hierarchy changes, don't forget to update
  * Lib/test/exception_hierarchy.txt
  */
@@ -79,7 +89,13 @@ BaseException_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
 
 static inline void BaseException_init_timestamp(PyBaseExceptionObject *self)
 {
-    PyTime_TimeRaw(&self->timestamp_ns);  /* fills in 0 on failure. */
+    if (!should_collect_traceback_timestamps() ||
+        Py_IS_TYPE(self, (PyTypeObject *)PyExc_StopIteration) ||
+        Py_IS_TYPE(self, (PyTypeObject *)PyExc_StopAsyncIteration)) {
+        self->timestamp_ns = 0;  /* fast; frequent non-error control flow. */
+    } else {
+        PyTime_TimeRaw(&self->timestamp_ns);  /* fills in 0 on failure. */
+    }
 }
 
 static int
@@ -89,12 +105,7 @@ BaseException_init(PyBaseExceptionObject *self, PyObject *args, PyObject *kwds)
         return -1;
 
     Py_XSETREF(self->args, Py_NewRef(args));
-    if (Py_IS_TYPE(self, (PyTypeObject *)PyExc_StopIteration) ||
-        Py_IS_TYPE(self, (PyTypeObject *)PyExc_StopAsyncIteration)) {
-        self->timestamp_ns = 0;  /* fast; frequent non-error control flow. */
-    } else {
-        BaseException_init_timestamp(self);
-    }
+    BaseException_init_timestamp(self);
     return 0;
 }
 
@@ -117,7 +128,7 @@ BaseException_vectorcall(PyObject *type_obj, PyObject * const*args,
     // The dict is created on the fly in PyObject_GenericSetAttr()
     self->dict = NULL;
     self->notes = NULL;
-    BaseException_init_timestamp(self);
+    BaseException_init_timestamp(self);  // self.timestamp_ns = ...
     self->traceback = NULL;
     self->cause = NULL;
     self->context = NULL;
@@ -4429,4 +4440,3 @@ _PyException_AddNote(PyObject *exc, PyObject *note)
     Py_XDECREF(r);
     return res;
 }
-

Python/initconfig.c

@@ -1943,7 +1943,7 @@ normalize_timestamp_format(const wchar_t *value)
         return L"us";
     }
     if (wcscmp(value, L"0") == 0) {
-        /* Treat "0" as empty string to disable the feature */
+        /* "0" means disable the feature. */
         return L"";
     }
     return value;
@@ -1963,7 +1963,8 @@ config_init_traceback_timestamps(PyConfig *config)
         /* For environment variables, silently ignore invalid values */
         if (is_valid_timestamp_format(wenv)) {
             const wchar_t *normalized = normalize_timestamp_format(wenv);
-            PyStatus status = PyConfig_SetString(config, &config->traceback_timestamps, normalized);
+            PyStatus status = PyConfig_SetString(
+                config, &config->traceback_timestamps, normalized);
             PyMem_RawFree(wenv);
             if (_PyStatus_EXCEPTION(status)) {
                 return status;
@@ -1974,21 +1975,24 @@ config_init_traceback_timestamps(PyConfig *config)
     }
 
     /* -X option overrides environment variable */
-    const wchar_t *xoption = config_get_xoption_value(config, L"traceback_timestamps");
+    const wchar_t *xoption = config_get_xoption_value(
+        config, L"traceback_timestamps");
     if (xoption != NULL) {
-        /* If value is empty (just -X traceback_timestamps with no =), use "us" as default */
+        /* If just -X traceback_timestamps with no =, use "us" as default */
         const wchar_t *value = (*xoption != '\0') ? xoption : L"us";
 
         /* Validate command line option values, error out if invalid */
         if (is_valid_timestamp_format(value)) {
             const wchar_t *normalized = normalize_timestamp_format(value);
-            PyStatus status = PyConfig_SetString(config, &config->traceback_timestamps, normalized);
+            PyStatus status = PyConfig_SetString(
+                config, &config->traceback_timestamps, normalized);
             if (_PyStatus_EXCEPTION(status)) {
                 return status;
             }
         } else {
-            return PyStatus_Error("Invalid value for -X traceback_timestamps option. "
-                                 "Valid values are: us, ns, iso, 0, 1 or empty.");
+            return PyStatus_Error(
+                "Invalid -X traceback_timestamps=value option.  Valid "
+                "values are: us, ns, iso, 0, 1 or empty.");
         }
     }