Setting the __traceback__ attribute in future.utils.raise_from - close #340

Author: Lucas-C

TESTING.txt

@@ -7,3 +7,11 @@ The test suite can be run either with:
 which uses the unittest module's test discovery mechanism, or with:
 
     $ py.test
+
+To execute a single test:
+
+    $ python setup.py test -s tests.test_future.test_utils.TestCause.test_chained_exceptions_stacktrace
+
+or with:
+
+    $ pytest -k test_chained_exceptions_stacktrace

src/future/utils/__init__.py

@@ -441,12 +441,14 @@ def raise_from(exc, cause):
         e.__suppress_context__ = False
         if isinstance(cause, type) and issubclass(cause, Exception):
             e.__cause__ = cause()
+            e.__cause__.__traceback__ = sys.exc_info()[2]
             e.__suppress_context__ = True
         elif cause is None:
             e.__cause__ = None
             e.__suppress_context__ = True
         elif isinstance(cause, BaseException):
             e.__cause__ = cause
+            e.__cause__.__traceback__ = sys.exc_info()[2]
             e.__suppress_context__ = True
         else:
             raise TypeError("exception causes must derive from BaseException")

tests/test_future/test_utils.py

@@ -4,7 +4,7 @@
 """
 
 from __future__ import absolute_import, unicode_literals, print_function
-import sys
+import sys, traceback
 from future.builtins import *
 from future.utils import (old_div, istext, isbytes, native, PY2, PY3,
                          native_str, raise_, as_native_str, ensure_new_type,
@@ -314,6 +314,61 @@ def __init__(self):
         else:
             self.fail("No exception raised")
 
+    def test_single_exception_stacktrace(self):
+        expected = '''Traceback (most recent call last):
+  File "/opt/python-future/tests/test_future/test_utils.py", line 328, in test_single_exception_stacktrace
+    raise CustomException('ERROR')
+'''
+        if PY2:
+            expected += 'CustomException: ERROR\n'
+        else:
+            expected += 'tests.test_future.test_utils.CustomException: ERROR\n'
+
+        try:
+            raise CustomException('ERROR')
+        except:
+            self.assertEqual(expected, traceback.format_exc())
+        else:
+            self.fail('No exception raised')
+
+    if PY2:
+        def test_chained_exceptions_stacktrace(self):
+            expected = '''Traceback (most recent call last):
+  File "/opt/python-future/tests/test_future/test_utils.py", line 354, in test_chained_exceptions_stacktrace
+    raise_from(CustomException('ERROR'), val_err)
+  File "/opt/python-future/src/future/utils/__init__.py", line 456, in raise_from
+    raise e
+CustomException: ERROR
+
+The above exception was the direct cause of the following exception:
+
+  File "/opt/python-future/tests/test_future/test_utils.py", line 352, in test_chained_exceptions_stacktrace
+    raise ValueError('Wooops')
+ValueError: Wooops
+'''
+
+            try:
+                try:
+                    raise ValueError('Wooops')
+                except ValueError as val_err:
+                    raise_from(CustomException('ERROR'), val_err)
+            except Exception as err:
+                self.assertEqual(expected.splitlines(), traceback.format_exc().splitlines())
+            else:
+                self.fail('No exception raised')
+
+
+class CustomException(Exception):
+    if PY2:
+        def __str__(self):
+            out = Exception.__str__(self)
+            if hasattr(self, '__cause__') and self.__cause__ and hasattr(self.__cause__, '__traceback__') and self.__cause__.__traceback__:
+                out += '\n\nThe above exception was the direct cause of the following exception:\n\n'
+                out += ''.join(traceback.format_tb(self.__cause__.__traceback__) + ['{}: {}'.format(self.__cause__.__class__.__name__, self.__cause__)])
+            return out
+    else:
+        pass
+
 
 if __name__ == '__main__':
     unittest.main()