code: add ExceptionInfo.from_exception

The old-style `sys.exc_info()` triplet is redundant nowadays with
`(type(exc), exc, exc.__traceback__)`, and is beginning to get
soft-deprecated in Python 3.12.

Add a nicer API to ExceptionInfo which takes just the exc instead of the
triplet. There are already a few internal uses which benefit.

changelog/10901.feature.rst

@@ -0,0 +1,2 @@
+Added :func:`ExceptionInfo.from_exception() <pytest.ExceptionInfo.from_exception>`, a simpler way to create an :class:`~pytest.ExceptionInfo` from an exception.
+This can replace :func:`ExceptionInfo.from_exc_info() <pytest.ExceptionInfo.from_exc_info()>` for most uses.

src/_pytest/_code/code.py

@@ -469,22 +469,41 @@ class ExceptionInfo(Generic[E]):
         self._traceback = traceback
 
     @classmethod
-    def from_exc_info(
+    def from_exception(
         cls,
-        exc_info: Tuple[Type[E], E, TracebackType],
+        # Ignoring error: "Cannot use a covariant type variable as a parameter".
+        # This is OK to ignore because this class is (conceptually) readonly.
+        # See https://github.com/python/mypy/issues/7049.
+        exception: E,  # type: ignore[misc]
         exprinfo: Optional[str] = None,
     ) -> "ExceptionInfo[E]":
-        """Return an ExceptionInfo for an existing exc_info tuple.
+        """Return an ExceptionInfo for an existing exception.
 
-        .. warning::
+        The exception must have a non-``None`` ``__traceback__`` attribute,
-
+        otherwise this function fails with an assertion error. This means that
-            Experimental API
+        the exception must have been raised, or added a traceback with the
+        :py:meth:`~BaseException.with_traceback()` method.
 
         :param exprinfo:
             A text string helping to determine if we should strip
             ``AssertionError`` from the output. Defaults to the exception
             message/``__str__()``.
+
+        .. versionadded:: 7.4
         """
+        assert (
+            exception.__traceback__
+        ), "Exceptions passed to ExcInfo.from_exception(...) must have a non-None __traceback__."
+        exc_info = (type(exception), exception, exception.__traceback__)
+        return cls.from_exc_info(exc_info, exprinfo)
+
+    @classmethod
+    def from_exc_info(
+        cls,
+        exc_info: Tuple[Type[E], E, TracebackType],
+        exprinfo: Optional[str] = None,
+    ) -> "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)
@@ -965,21 +984,13 @@ class FormattedExcinfo:
 
             if e.__cause__ is not None and self.chain:
                 e = e.__cause__
-                excinfo_ = (
+                excinfo_ = ExceptionInfo.from_exception(e) if e.__traceback__ else None
-                    ExceptionInfo.from_exc_info((type(e), e, e.__traceback__))
-                    if e.__traceback__
-                    else None
-                )
                 descr = "The above exception was the direct cause of the following exception:"
             elif (
                 e.__context__ is not None and not e.__suppress_context__ and self.chain
             ):
                 e = e.__context__
-                excinfo_ = (
+                excinfo_ = ExceptionInfo.from_exception(e) if e.__traceback__ else None
-                    ExceptionInfo.from_exc_info((type(e), e, e.__traceback__))
-                    if e.__traceback__
-                    else None
-                )
                 descr = "During handling of the above exception, another exception occurred:"
             else:
                 e = None

src/_pytest/python_api.py

@@ -950,11 +950,7 @@ def raises(  # noqa: F811
         try:
             func(*args[1:], **kwargs)
         except expected_exception as e:
-            # We just caught the exception - there is a traceback.
+            return _pytest._code.ExceptionInfo.from_exception(e)
-            assert e.__traceback__ is not None
-            return _pytest._code.ExceptionInfo.from_exc_info(
-                (type(e), e, e.__traceback__)
-            )
     fail(message)
 
 

testing/code/test_excinfo.py

@@ -53,6 +53,20 @@ def test_excinfo_from_exc_info_simple() -> None:
     assert info.type == ValueError
 
 
+def test_excinfo_from_exception_simple() -> None:
+    try:
+        raise ValueError
+    except ValueError as e:
+        assert e.__traceback__ is not None
+        info = _pytest._code.ExceptionInfo.from_exception(e)
+    assert info.type == ValueError
+
+
+def test_excinfo_from_exception_missing_traceback_assertion() -> None:
+    with pytest.raises(AssertionError, match=r"must have.*__traceback__"):
+        _pytest._code.ExceptionInfo.from_exception(ValueError())
+
+
 def test_excinfo_getstatement():
     def g():
         raise ValueError