Add AssertRaisesContext.exc_val

Author: srittau

NEWS.md

@@ -19,6 +19,15 @@ with assert_raises(KeyError) as context:
     ...
 ```  
 
+* Add `AssertRaisesContext.exc_val` property to access the caught
+  exception after leaving the context manager:
+
+```python
+with assert_raises(KeyError) as context:
+    ...
+assert_equal("expected message", str(context.exc_val))
+```  
+
 News in asserts 0.9.1
 =====================
 

asserts/__init__.py

@@ -880,16 +880,18 @@ def __init__(self, exception, msg_fmt="{msg}"):
         self.exception = exception
         self.msg_fmt = msg_fmt
         self._exc_type = exception
+        self._exc_val = None
         self._exception_name = getattr(exception, "__name__", str(exception))
         self._tests = []
 
     def __enter__(self):
         return self
 
     def __exit__(self, exc_type, exc_val, exc_tb):
-        if not exc_type:
+        if not exc_type or not exc_val:
             msg = "{} not raised".format(self._exception_name)
             fail(self.format_message(msg))
+        self._exc_val = exc_val
         if not issubclass(exc_type, self.exception):
             return False
         for test in self._tests:
@@ -913,6 +915,12 @@ class was raised. The callback will get the raised exception as only
         """
         self._tests.append(cb)
 
+    @property
+    def exc_val(self):
+        if self._exc_val is None:
+            raise RuntimeError("must be called after leaving the context")
+        return self._exc_val
+
 
 class AssertRaisesRegexContext(AssertRaisesContext):
 

asserts/__init__.pyi

@@ -31,6 +31,8 @@ class AssertRaisesContext(Generic[_E]):
     ) -> bool: ...
     def format_message(self, default_msg: Text) -> Text: ...
     def add_test(self, cb: Callable[[_E], Any]) -> None: ...
+    @property
+    def exc_val(self) -> _E: ...
 
 class AssertRaisesErrnoContext(AssertRaisesContext[_E]):
     expected_errno: int

test_asserts.py

@@ -936,6 +936,17 @@ def test_assert_raises__raises_right_exception(self):
         with assert_raises(KeyError):
             raise KeyError()
 
+    def test_assert_raises__exc_val(self):
+        exc = KeyError()
+        with assert_raises(KeyError) as context:
+            raise exc
+        assert_is(exc, context.exc_val)
+
+    def test_assert_raises__exc_val_within_context(self):
+        with assert_raises(RuntimeError):
+            with assert_raises(KeyError) as context:
+                context.exc_val
+
     def test_assert_raises__raises_subclass(self):
         class MyError(IndexError):
             pass