Exception chains can be navigated when dropped into Pdb in Python 3.13+ (#12708)

Closes #12707

Author: AWhetter

AUTHORS

@@ -51,6 +51,7 @@ Aron Coyle
 Aron Curzon
 Arthur Richard
 Ashish Kurmi
+Ashley Whetter
 Aviral Verma
 Aviv Palivoda
 Babak Keyvani

changelog/12707.improvement.rst

@@ -0,0 +1 @@
+Exception chains can be navigated when dropped into Pdb in Python 3.13+.

src/_pytest/debugging.py

@@ -292,8 +292,8 @@ def pytest_exception_interact(
             _enter_pdb(node, call.excinfo, report)
 
     def pytest_internalerror(self, excinfo: ExceptionInfo[BaseException]) -> None:
-        tb = _postmortem_traceback(excinfo)
-        post_mortem(tb)
+        exc_or_tb = _postmortem_exc_or_tb(excinfo)
+        post_mortem(exc_or_tb)
 
 
 class PdbTrace:
@@ -354,32 +354,46 @@ def _enter_pdb(
     tw.sep(">", "traceback")
     rep.toterminal(tw)
     tw.sep(">", "entering PDB")
-    tb = _postmortem_traceback(excinfo)
+    tb_or_exc = _postmortem_exc_or_tb(excinfo)
     rep._pdbshown = True  # type: ignore[attr-defined]
-    post_mortem(tb)
+    post_mortem(tb_or_exc)
     return rep
 
 
-def _postmortem_traceback(excinfo: ExceptionInfo[BaseException]) -> types.TracebackType:
+def _postmortem_exc_or_tb(
+    excinfo: ExceptionInfo[BaseException],
+) -> types.TracebackType | BaseException:
     from doctest import UnexpectedException
 
+    get_exc = sys.version_info >= (3, 13)
     if isinstance(excinfo.value, UnexpectedException):
         # A doctest.UnexpectedException is not useful for post_mortem.
         # Use the underlying exception instead:
-        return excinfo.value.exc_info[2]
+        underlying_exc = excinfo.value
+        if get_exc:
+            return underlying_exc.exc_info[1]
+
+        return underlying_exc.exc_info[2]
     elif isinstance(excinfo.value, ConftestImportFailure):
         # A config.ConftestImportFailure is not useful for post_mortem.
         # Use the underlying exception instead:
-        assert excinfo.value.cause.__traceback__ is not None
-        return excinfo.value.cause.__traceback__
+        cause = excinfo.value.cause
+        if get_exc:
+            return cause
+
+        assert cause.__traceback__ is not None
+        return cause.__traceback__
     else:
         assert excinfo._excinfo is not None
+        if get_exc:
+            return excinfo._excinfo[1]
+
         return excinfo._excinfo[2]
 
 
-def post_mortem(t: types.TracebackType) -> None:
+def post_mortem(tb_or_exc: types.TracebackType | BaseException) -> None:
     p = pytestPDB._init_pdb("post_mortem")
     p.reset()
-    p.interaction(None, t)
+    p.interaction(None, tb_or_exc)
     if p.quitting:
         outcomes.exit("Quitting debugger")

testing/test_debugging.py

@@ -103,7 +103,10 @@ def test_func():
         )
         assert rep.failed
         assert len(pdblist) == 1
-        tb = _pytest._code.Traceback(pdblist[0][0])
+        if sys.version_info < (3, 13):
+            tb = _pytest._code.Traceback(pdblist[0][0])
+        else:
+            tb = _pytest._code.Traceback(pdblist[0][0].__traceback__)
         assert tb[-1].name == "test_func"
 
     def test_pdb_on_xfail(self, pytester: Pytester, pdblist) -> None:
@@ -921,6 +924,39 @@ def test_foo():
         child.expect("custom set_trace>")
         self.flush(child)
 
+    @pytest.mark.skipif(
+        sys.version_info < (3, 13),
+        reason="Navigating exception chains was introduced in 3.13",
+    )
+    def test_pdb_exception_chain_navigation(self, pytester: Pytester) -> None:
+        p1 = pytester.makepyfile(
+            """
+            def inner_raise():
+                is_inner = True
+                raise RuntimeError("Woops")
+
+            def outer_raise():
+                is_inner = False
+                try:
+                    inner_raise()
+                except RuntimeError:
+                    raise RuntimeError("Woopsie")
+
+            def test_1():
+                outer_raise()
+                assert True
+        """
+        )
+        child = pytester.spawn_pytest(f"--pdb {p1}")
+        child.expect("Pdb")
+        child.sendline("is_inner")
+        child.expect_exact("False")
+        child.sendline("exceptions 0")
+        child.sendline("is_inner")
+        child.expect_exact("True")
+        child.sendeof()
+        self.flush(child)
+
 
 class TestDebuggingBreakpoints:
     @pytest.mark.parametrize("arg", ["--pdb", ""])