[tune] Use traceback.format_tb() (fixes #5135) (#5136)

python/ray/tune/function_runner.py

@@ -3,10 +3,10 @@ from __future__ import division
 from __future__ import print_function
 
 import logging
-import sys
 import time
 import inspect
 import threading
+import traceback
 from six.moves import queue
 
 from ray.tune import track
@@ -100,12 +100,9 @@ class _RunnerThread(threading.Thread):
                 # report the error but avoid indefinite blocking which would
                 # prevent the exception from being propagated in the unlikely
                 # case that something went terribly wrong
-                err_type, err_value, err_tb = sys.exc_info()
-                err_tb = err_tb.format_exc()
+                err_tb_str = traceback.format_exc()
                 self._error_queue.put(
-                    (err_type, err_value, err_tb),
-                    block=True,
-                    timeout=ERROR_REPORT_TIMEOUT)
+                    err_tb_str, block=True, timeout=ERROR_REPORT_TIMEOUT)
             except queue.Full:
                 logger.critical(
                     ("Runner Thread was unable to report error to main "
@@ -234,13 +231,10 @@ class FunctionRunner(Trainable):
 
     def _report_thread_runner_error(self, block=False):
         try:
-            err_type, err_value, err_tb = self._error_queue.get(
+            err_tb_str = self._error_queue.get(
                 block=block, timeout=ERROR_FETCH_TIMEOUT)
-            raise TuneError(("Trial raised a {err_type} exception with value: "
-                             "{err_value}\nWith traceback:\n{err_tb}").format(
-                                 err_type=err_type,
-                                 err_value=err_value,
-                                 err_tb=err_tb))
+            raise TuneError(("Trial raised an exception. Traceback:\n{}"
+                             .format(err_tb_str)))
         except queue.Empty:
             pass