result: add result.force_exception API

Hookwrappers do not currently provide a public, proper way to override
or force an exception. Raising directly from the hookwrapper causes
further wrappers to be skipped, but this is not something we want to
change at this point, for fear of breaking things, and because we are
adding a replacement. See pytest-dev#244 on this issue.

changelog/394.feature.rst

@@ -0,0 +1,5 @@
+Added the :meth:`~pluggy._callers._Result.force_exception` method to ``_Result``.
+
+``force_exception`` allows hookwrappers to force an exception or override/adjust an existing exception of a hook invocation,
+in a properly behaving manner. Using ``force_exception`` is preferred over raising an exception from the hookwrapper,
+because raising an exception causes other hookwrappers to be skipped.

docs/api_reference.rst

@@ -10,6 +10,7 @@ API Reference
 .. autoclass:: pluggy._callers._Result
 .. automethod:: pluggy._callers._Result.get_result
 .. automethod:: pluggy._callers._Result.force_result
+.. automethod:: pluggy._callers._Result.force_exception
 
 .. autoclass:: pluggy._hooks._HookCaller
 .. automethod:: pluggy._hooks._HookCaller.call_extra

docs/index.rst

@@ -387,7 +387,13 @@ be implemented as generator function with a single ``yield`` in its body:
         # all corresponding hookimpls are invoked here
         outcome = yield
 
-        for item in outcome.get_result():
+        try:
+            result = outcome.get_result()
+        except BaseException as e:
+            outcome.force_exception(e)
+            return
+
+        for item in result:
             print("JSON config override is {}".format(item))
 
         if config.debug:
@@ -397,14 +403,21 @@ be implemented as generator function with a single ``yield`` in its body:
             outcome.force_result(defaults)
 
 The generator is :py:meth:`sent <python:generator.send>` a :py:class:`pluggy._callers._Result` object which can
-be assigned in the ``yield`` expression and used to override or inspect
-the final result(s) returned back to the caller using the
-:py:meth:`~pluggy._callers._Result.force_result` or
-:py:meth:`~pluggy._callers._Result.get_result` methods.
+be assigned in the ``yield`` expression and used to inspect
+the final result(s) or exceptions returned back to the caller using the
+:py:meth:`~pluggy._callers._Result.get_result` method, override the result
+using the :py:meth:`~pluggy._callers._Result.force_result`, or override
+the exception using the :py:meth:`~pluggy._callers._Result.force_exception`
+method.
 
 .. note::
-    Hook wrappers can **not** return results (as per generator function
-    semantics); they can only modify them using the ``_Result`` API.
+    Hookwrappers can **not** return results; they can only modify them using
+    the :py:meth:`~pluggy._callers._Result.force_result` API.
+
+    Hookwrappers should **not** raise exceptions; this will cause further
+    hookwrappers to be skipped. They should use
+    :py:meth:`~pluggy._callers._Result.force_exception` to adjust the
+    exception.
 
 Also see the :ref:`pytest:hookwrapper` section in the ``pytest`` docs.
 

src/pluggy/_result.py

@@ -75,10 +75,22 @@ def force_result(self, result: _T) -> None:
         If the hook was marked as a ``firstresult`` a single value should
         be set, otherwise set a (modified) list of results. Any exceptions
         found during invocation will be deleted.
+
+        This overrides any previous result or exception.
         """
         self._result = result
         self._exception = None
 
+    def force_exception(self, exception: BaseException) -> None:
+        """Force the result to fail with ``exception``.
+
+        This overrides any previous result or exception.
+
+        .. versionadded:: 1.1.0
+        """
+        self._result = None
+        self._exception = exception
+
     def get_result(self) -> _T:
         """Get the result(s) for this hook call.
 

testing/test_multicall.py

@@ -161,3 +161,52 @@ def m2():
     with pytest.raises(exc):
         MC([m2, m1], {})
     assert out == ["m1 init", "m1 finish"]
+
+
+def test_hookwrapper_force_exception() -> None:
+    out = []
+
+    @hookimpl(hookwrapper=True)
+    def m1():
+        out.append("m1 init")
+        result = yield
+        try:
+            result.get_result()
+        except BaseException as exc:
+            result.force_exception(exc)
+        out.append("m1 finish")
+
+    @hookimpl(hookwrapper=True)
+    def m2():
+        out.append("m2 init")
+        result = yield
+        try:
+            result.get_result()
+        except BaseException as exc:
+            new_exc = OSError("m2")
+            new_exc.__cause__ = exc
+            result.force_exception(new_exc)
+        out.append("m2 finish")
+
+    @hookimpl(hookwrapper=True)
+    def m3():
+        out.append("m3 init")
+        yield
+        out.append("m3 finish")
+
+    @hookimpl
+    def m4():
+        raise ValueError("m4")
+
+    with pytest.raises(OSError, match="m2") as excinfo:
+        MC([m4, m3, m2, m1], {})
+    assert out == [
+        "m1 init",
+        "m2 init",
+        "m3 init",
+        "m3 finish",
+        "m2 finish",
+        "m1 finish",
+    ]
+    assert excinfo.value.__cause__ is not None
+    assert str(excinfo.value.__cause__) == "m4"