Fix ExceptionInfo.for_later() not populating _striptext

changelog/12175.bugfix.rst

@@ -0,0 +1 @@
+Fixed :meth:`ExceptionInfo.exconly(tryshort=True) <ExceptionInfo.exconly>` not stripping the ``AssertionError`` prefix when the :class:`ExceptionInfo` was created via :meth:`ExceptionInfo.for_later` (e.g. when using :func:`pytest.raises`).

src/_pytest/_code/code.py

@@ -545,6 +545,20 @@ def from_exception(
         exc_info = (type(exception), exception, exception.__traceback__)
         return cls.from_exc_info(exc_info, exprinfo)
 
+    @classmethod
+    def _compute_striptext(
+        cls,
+        exc_info: tuple[type[E], E, TracebackType],
+    ) -> str:
+        """Determine if AssertionError prefix should be stripped from output."""
+        if isinstance(exc_info[1], AssertionError):
+            exprinfo = getattr(exc_info[1], "msg", None)
+            if exprinfo is None:
+                exprinfo = saferepr(exc_info[1])
+            if exprinfo and exprinfo.startswith(cls._assert_start_repr):
+                return "AssertionError: "
+        return ""
+
     @classmethod
     def from_exc_info(
         cls,
@@ -553,12 +567,8 @@ def from_exc_info(
     ) -> ExceptionInfo[E]:
         """Like :func:`from_exception`, but using old-style exc_info tuple."""
         _striptext = ""
-        if exprinfo is None and isinstance(exc_info[1], AssertionError):
-            exprinfo = getattr(exc_info[1], "msg", None)
-            if exprinfo is None:
-                exprinfo = saferepr(exc_info[1])
-            if exprinfo and exprinfo.startswith(cls._assert_start_repr):
-                _striptext = "AssertionError: "
+        if exprinfo is None:
+            _striptext = cls._compute_striptext(exc_info)
 
         return cls(exc_info, _striptext, _ispytest=True)
 
@@ -591,6 +601,7 @@ def fill_unfilled(self, exc_info: tuple[type[E], E, TracebackType]) -> None:
         """Fill an unfilled ExceptionInfo created with ``for_later()``."""
         assert self._excinfo is None, "ExceptionInfo was already filled"
         self._excinfo = exc_info
+        self._striptext = self._compute_striptext(exc_info)
 
     @property
     def type(self) -> type[E]:

testing/code/test_excinfo.py

@@ -365,6 +365,51 @@ def test_excinfo_for_later() -> None:
     assert "for raises" in str(e)
 
 
+def test_excinfo_for_later_strips_assertion(pytester: Pytester) -> None:
+    """ExceptionInfo created via for_later() should strip AssertionError prefix
+    with exconly(tryshort=True) for rewritten assertions (#12175)."""
+    pytester.makepyfile(
+        """
+        import pytest
+
+        def test_tryshort():
+            with pytest.raises(AssertionError) as exc_info:
+                assert 1 == 2
+            # tryshort should strip 'AssertionError: ' from rewritten assertions
+            result = exc_info.exconly(tryshort=True)
+            assert not result.startswith("AssertionError"), (
+                f"Expected stripped prefix, got: {result!r}"
+            )
+        """
+    )
+    result = pytester.runpytest()
+    result.assert_outcomes(passed=1)
+
+
+def test_excinfo_for_later_no_strip_non_assertion() -> None:
+    """fill_unfilled() should not strip prefix for non-AssertionError exceptions."""
+    excinfo: ExceptionInfo[ValueError] = ExceptionInfo.for_later()
+    try:
+        raise ValueError("test error")
+    except ValueError:
+        excinfo.fill_unfilled(sys.exc_info())  # type: ignore[arg-type]
+    assert excinfo.exconly(tryshort=True).startswith("ValueError")
+
+
+def test_excinfo_for_later_strips_manual_assertion() -> None:
+    """fill_unfilled() handles AssertionError with explicit .msg attribute."""
+    excinfo: ExceptionInfo[AssertionError] = ExceptionInfo.for_later()
+    try:
+        err = AssertionError("manual error")
+        err.msg = "assert something"  # type: ignore[attr-defined]
+        raise err
+    except AssertionError:
+        excinfo.fill_unfilled(sys.exc_info())  # type: ignore[arg-type]
+    # Manual .msg that doesn't match _assert_start_repr should not strip
+    result = excinfo.exconly(tryshort=True)
+    assert result.startswith("AssertionError")
+
+
 def test_excinfo_errisinstance():
     excinfo = pytest.raises(ValueError, h)
     assert excinfo.errisinstance(ValueError)