Re-add removed compat helpers

These were dropped in 7d62575
but that should have resulted in a major release since it was
an API break. Re-add them temporarily until we release a v3.0.0.

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

tests/test_compat.py

@@ -5,9 +5,11 @@
 import ast
 import io
 import sys
+import traceback
 
 import testtools
 from testtools.compat import (
+    reraise,
     text_repr,
     unicode_output_stream,
 )
@@ -216,6 +218,43 @@ def test_unicode_examples_multiline(self):
             self.assertEqual(ast.literal_eval(actual), u)
 
 
+class TestReraise(testtools.TestCase):
+    """Tests for trivial reraise wrapper needed for Python 2/3 changes"""
+
+    def test_exc_info(self):
+        """After reraise exc_info matches plus some extra traceback"""
+        try:
+            raise ValueError("Bad value")
+        except ValueError:
+            _exc_info = sys.exc_info()
+        try:
+            reraise(*_exc_info)
+        except ValueError:
+            _new_exc_info = sys.exc_info()
+        self.assertIs(_exc_info[0], _new_exc_info[0])
+        self.assertIs(_exc_info[1], _new_exc_info[1])
+        expected_tb = traceback.extract_tb(_exc_info[2])
+        self.assertEqual(
+            expected_tb, traceback.extract_tb(_new_exc_info[2])[-len(expected_tb) :]
+        )
+
+    def test_custom_exception_no_args(self):
+        """Reraising does not require args attribute to contain params"""
+
+        class CustomException(Exception):
+            """Exception that expects and sets attrs but not args"""
+
+            def __init__(self, value):
+                Exception.__init__(self)
+                self.value = value
+
+        try:
+            raise CustomException("Some value")
+        except CustomException:
+            _exc_info = sys.exc_info()
+        self.assertRaises(CustomException, reraise, *_exc_info)
+
+
 def test_suite():
     from unittest import TestLoader
 

testtools/compat.py

@@ -12,9 +12,45 @@
 import codecs
 import io
 import sys
+import types
 import unicodedata
+import warnings
 from io import BytesIO, StringIO  # for backwards-compat
-from typing import IO
+from typing import IO, Any, NoReturn
+
+
+def reraise(
+    exc_class: type[BaseException],
+    exc_obj: BaseException,
+    exc_tb: types.TracebackType,
+    _marker: Any = object(),
+) -> NoReturn:
+    """Re-raise an exception received from sys.exc_info() or similar."""
+    warnings.warn(
+        "This is not necessary in Python 3.",
+        DeprecationWarning,
+        stacklevel=2,
+    )
+    raise exc_obj.with_traceback(exc_tb)
+
+
+def _u(s: str) -> str:
+    warnings.warn(
+        "This is not necessary in Python 3.",
+        DeprecationWarning,
+        stacklevel=2,
+    )
+    return s
+
+
+def _b(s: str) -> bytes:
+    """A byte literal."""
+    warnings.warn(
+        "This is not necessary in Python 3.",
+        DeprecationWarning,
+        stacklevel=2,
+    )
+    return s.encode("latin-1")
 
 
 def _slow_escape(text: str) -> str: