add raise_from (fixes #102)

Patch from Robert Collins.

4 files changed, 39 insertions, 0 deletions

diff --git a/CHANGES b/CHANGES
index fa74e77..cae8ef0 100644
--- a/CHANGES
+++ b/CHANGES
@@ -6,6 +6,8 @@ This file lists the changes in each six version.
 Development version
 -------------------
 
+- Issue #102: Add `raise_from` to abstract out Python 3's raise from syntax.
+
 - Issue #97: Optimize `six.iterbytes` on Python 2.
 
 - Issue #98: Fix `six.moves` race condition in multi-threaded code.
diff --git a/documentation/index.rst b/documentation/index.rst
index c7ca996..7851421 100644
--- a/documentation/index.rst
+++ b/documentation/index.rst
@@ -282,6 +282,13 @@ Python 2 and 3.
       ok. :)
 
 
+.. function:: raise_from(exc_value, exc_value_from)
+
+   Raise an exception from a context.  On Python 3, this is equivalent to
+   ``raise exc_value from exc_value_from``.  On Python 2, which does not support
+   exception chaining, it is equivalent to ``raise exc_value``.
+
+
 .. function:: reraise(exc_type, exc_value, exc_traceback=None)
 
    Reraise an exception, possibly with a different traceback.  In the simple
diff --git a/six.py b/six.py
index 232a0c7..a8cdb22 100644
--- a/six.py
+++ b/six.py
@@ -659,6 +659,15 @@ else:
 """)
 
 
+if sys.version_info > (3, 2):
+    exec_("""def raise_from(value, from_value):
+    raise value from from_value
+""")
+else:
+    def raise_from(value, from_value):
+        raise value
+
+
 print_ = getattr(moves.builtins, "print", None)
 if print_ is None:
     def print_(*args, **kwargs):
diff --git a/test_six.py b/test_six.py
index 4163520..b0ccd8d 100644
--- a/test_six.py
+++ b/test_six.py
@@ -588,6 +588,27 @@ def test_reraise():
         assert tb is get_next(tb2)
 
 
+def test_raise_from():
+    try:
+        try:
+            raise Exception("blah")
+        except Exception:
+            ctx = sys.exc_info()[1]
+            f = Exception("foo")
+            six.raise_from(f, None)
+    except Exception:
+        tp, val, tb = sys.exc_info()
+    if sys.version_info[:2] > (3, 0):
+        # We should have done a raise f from None equivalent.
+        assert val.__cause__ is None
+        assert val.__context__ is ctx
+    if sys.version_info[:2] >= (3, 3):
+        # And that should suppress the context on the exception.
+        assert val.__suppress_context__
+    # For all versions the outer exception should have raised successfully.
+    assert str(val) == "foo"
+
+
 def test_print_():
     save = sys.stdout
     out = sys.stdout = six.moves.StringIO()