Cleaner tracebacks for captured exceptions

Author: RazerM

newsfragments/21.feature.rst

@@ -0,0 +1,2 @@
+On Python 3, the exception frame generated within :func:`capture` and
+:func:`acapture` has been removed from the traceback.

src/outcome/_async.py

@@ -3,6 +3,7 @@
 from ._sync import Error as ErrorBase
 from ._sync import Outcome as OutcomeBase
 from ._sync import Value as ValueBase
+from ._util import remove_tb_frames
 
 __all__ = ['Error', 'Outcome', 'Value', 'acapture', 'capture']
 
@@ -18,6 +19,7 @@ def capture(sync_fn, *args, **kwargs):
     try:
         return Value(sync_fn(*args, **kwargs))
     except BaseException as exc:
+        exc = remove_tb_frames(exc, 1)
         return Error(exc)
 
 
@@ -31,6 +33,7 @@ async def acapture(async_fn, *args, **kwargs):
     try:
         return Value(await async_fn(*args, **kwargs))
     except BaseException as exc:
+        exc = remove_tb_frames(exc, 1)
         return Error(exc)
 
 

src/outcome/_sync.py

@@ -5,7 +5,7 @@
 
 import attr
 
-from ._util import ABC, AlreadyUsedError
+from ._util import ABC, AlreadyUsedError, remove_tb_frames
 
 __all__ = ['Error', 'Outcome', 'Value', 'capture']
 
@@ -20,6 +20,7 @@ def capture(sync_fn, *args, **kwargs):
     try:
         return Value(sync_fn(*args, **kwargs))
     except BaseException as exc:
+        exc = remove_tb_frames(exc, 1)
         return Error(exc)
 
 
@@ -104,7 +105,10 @@ def __repr__(self):
 
     def unwrap(self):
         self._set_unwrapped()
-        raise self.error
+        # Tracebacks show the 'raise' line below out of context, so let's give
+        # this variable a name that makes sense out of context.
+        captured_error = self.error
+        raise captured_error
 
     def send(self, it):
         self._set_unwrapped()

src/outcome/_util.py

@@ -24,6 +24,15 @@ def fix_one(obj):
         fix_one(obj)
 
 
+def remove_tb_frames(exc, n):
+    if sys.version_info < (3,):
+        return exc
+    tb = exc.__traceback__
+    for _ in range(n):
+        tb = tb.tb_next
+    return exc.with_traceback(tb)
+
+
 if sys.version_info < (3,):
 
     class ABC(object):

tests/test_async.py

@@ -1,4 +1,5 @@
 import sys
+import traceback
 
 import pytest
 import trio
@@ -50,3 +51,18 @@ async def my_agen_func():
         await e.asend(my_agen)
     with pytest.raises(StopAsyncIteration):
         await my_agen.asend(None)
+
+
+async def test_traceback_frame_removal():
+    async def raise_ValueError(x):
+        raise ValueError(x)
+
+    e = await outcome.acapture(raise_ValueError, 'abc')
+    try:
+        e.unwrap()
+    except Exception as exc:
+        frames = traceback.extract_tb(exc.__traceback__)
+        functions = [function for _, _, function, _ in frames]
+        assert functions[-2:] == ['unwrap', 'raise_ValueError']
+    else:
+        pytest.fail('Did not raise')

tests/test_sync.py

@@ -2,6 +2,7 @@
 from __future__ import absolute_import, division, print_function
 
 import sys
+import traceback
 
 import pytest
 
@@ -109,3 +110,19 @@ def raise_ValueError(x):
 def test_inheritance():
     assert issubclass(Value, outcome.Outcome)
     assert issubclass(Error, outcome.Outcome)
+
+
+@pytest.mark.skipif(sys.version_info < (3,), reason="requires python 3")
+def test_traceback_frame_removal():
+    def raise_ValueError(x):
+        raise ValueError(x)
+
+    e = outcome.capture(raise_ValueError, 'abc')
+    try:
+        e.unwrap()
+    except Exception as exc:
+        frames = traceback.extract_tb(exc.__traceback__)
+        functions = [function for _, _, function, _ in frames]
+        assert functions[-2:] == ['unwrap', 'raise_ValueError']
+    else:
+        pytest.fail('Did not raise')