Include original traceback when unwrapping an error (#57)

* Include original traceback when unwrapping an error.

* Only include traceback if failure is an exception.

* Address code review.

* Added a test.

* Happify lint.

Author: Omer Katz

returns/result.py

@@ -59,6 +59,9 @@ def value_or(self, default_value):
 
     def unwrap(self):
         """Raises an exception, since it does not have a value inside."""
+        if isinstance(self._inner_value, Exception):
+            raise UnwrapFailedError(self) from self._inner_value
+
         raise UnwrapFailedError(self)
 
     def failure(self):

tests/test_result/test_result_unwrap.py

@@ -15,3 +15,12 @@ def test_unwrap_failure():
     """Ensures that unwrap works for Failure monad."""
     with pytest.raises(UnwrapFailedError):
         assert Failure(5).unwrap()
+
+
+def test_unwrap_failure_with_exception():
+    """Ensures that unwrap raises from the original exception."""
+    expected_exception = ValueError('error')
+    with pytest.raises(UnwrapFailedError) as excinfo:
+        Failure(expected_exception).unwrap()
+
+    assert 'ValueError: error' in str(excinfo.getrepr())