Error.peek() raises a copy of the wrapped error

Author: graingert

newsfragments/47.feature.rst

@@ -1,3 +1,4 @@
 Remove reference to value/error when unwrapping outcome.
 Provide a ``.peek()`` method to recieve wrapped
-value/error without invalidating the outcome.
+value or a copy of the wrapped error
+without invalidating the outcome.

src/outcome/_impl.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import abc
+import copy
 from typing import (
     TYPE_CHECKING,
     AsyncGenerator,
@@ -125,10 +126,10 @@ class Outcome(abc.ABC, Generic[ValueT]):
 
     @abc.abstractmethod
     def peek(self) -> ValueT:
-        """Return or raise the contained value or exception, without
-        invalidating the outcome.
+        """Return the contained value or raise a copy of the contained
+        exception, without invalidating the outcome.
 
-        These two lines of code are equivalent::
+        These two lines of code are almost equivalent::
 
            x = fn(*args)
            x = outcome.capture(fn, *args).peek()
@@ -245,7 +246,7 @@ def _unwrap_error(self) -> BaseException:
     def peek(self) -> NoReturn:
         # 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
+        captured_error = copy.copy(self.error)
         try:
             raise captured_error
         finally:

tests/test_async.py

@@ -25,11 +25,13 @@ async def raise_ValueError(x):
     e = await outcome.acapture(raise_ValueError, 9)
     assert type(e.error) is ValueError
     assert e.error.args == (9,)
-    with pytest.raises(ValueError):
+    with pytest.raises(ValueError) as exc_info:
         e.peek()
-    with pytest.raises(ValueError):
+    with pytest.raises(ValueError) as exc_info2:
         e.unwrap()
 
+    assert exc_info.value is not exc_info2.value
+
 
 async def test_asend():
     async def my_agen_func():

tests/test_sync.py

@@ -24,10 +24,13 @@ def test_Outcome():
     e = Error(exc)
     assert e.error is exc
     assert repr(e) == f"Error({exc!r})"
-    with pytest.raises(RuntimeError):
+    with pytest.raises(RuntimeError) as exc_info:
         e.peek()
-    with pytest.raises(RuntimeError):
+    with pytest.raises(RuntimeError) as exc_info2:
         e.unwrap()
+
+    assert exc_info.value is not exc_info2.value
+
     with pytest.raises(AlreadyUsedError):
         e.unwrap()
     assert repr(e) == "Error(<AlreadyUsed>)"