nodes: change _prunetraceback to return the new traceback instead of modifying excinfo

This makes it usable as a general function, and just more understandable
in general.

src/_pytest/nodes.py

@@ -22,6 +22,7 @@ import _pytest._code
 from _pytest._code import getfslineno
 from _pytest._code.code import ExceptionInfo
 from _pytest._code.code import TerminalRepr
+from _pytest._code.code import Traceback
 from _pytest.compat import cached_property
 from _pytest.compat import LEGACY_PATH
 from _pytest.config import Config
@@ -432,8 +433,8 @@ class Node(metaclass=NodeMeta):
         assert current is None or isinstance(current, cls)
         return current
 
-    def _prunetraceback(self, excinfo: ExceptionInfo[BaseException]) -> None:
-        pass
+    def _traceback_filter(self, excinfo: ExceptionInfo[BaseException]) -> Traceback:
+        return excinfo.traceback
 
     def _repr_failure_py(
         self,
@@ -452,7 +453,7 @@ class Node(metaclass=NodeMeta):
         if self.config.getoption("fulltrace", False):
             style = "long"
         else:
-            self._prunetraceback(excinfo)
+            excinfo.traceback = self._traceback_filter(excinfo)
             if style == "auto":
                 style = "long"
         # XXX should excinfo.getrepr record all data and toterminal() process it?
@@ -554,13 +555,14 @@ class Collector(Node):
 
         return self._repr_failure_py(excinfo, style=tbstyle)
 
-    def _prunetraceback(self, excinfo: ExceptionInfo[BaseException]) -> None:
+    def _traceback_filter(self, excinfo: ExceptionInfo[BaseException]) -> Traceback:
         if hasattr(self, "path"):
             traceback = excinfo.traceback
             ntraceback = traceback.cut(path=self.path)
             if ntraceback == traceback:
                 ntraceback = ntraceback.cut(excludepath=tracebackcutdir)
-            excinfo.traceback = ntraceback.filter(excinfo)
+            return excinfo.traceback.filter(excinfo)
+        return excinfo.traceback
 
 
 def _check_initialpaths_for_relpath(session: "Session", path: Path) -> Optional[str]:

src/_pytest/python.py

@@ -1802,7 +1802,7 @@ class Function(PyobjMixin, nodes.Item):
     def setup(self) -> None:
         self._request._fillfixtures()
 
-    def _prunetraceback(self, excinfo: ExceptionInfo[BaseException]) -> None:
+    def _traceback_filter(self, excinfo: ExceptionInfo[BaseException]) -> Traceback:
         if hasattr(self, "_obj") and not self.config.getoption("fulltrace", False):
             code = _pytest._code.Code.from_function(get_real_func(self.obj))
             path, firstlineno = code.path, code.firstlineno
@@ -1814,19 +1814,22 @@ class Function(PyobjMixin, nodes.Item):
                     ntraceback = ntraceback.filter(filter_traceback)
                     if not ntraceback:
                         ntraceback = traceback
+            ntraceback = ntraceback.filter(excinfo)
 
-            excinfo.traceback = ntraceback.filter(excinfo)
             # issue364: mark all but first and last frames to
             # only show a single-line message for each frame.
             if self.config.getoption("tbstyle", "auto") == "auto":
-                if len(excinfo.traceback) > 2:
-                    excinfo.traceback = Traceback(
+                if len(ntraceback) > 2:
+                    ntraceback = Traceback(
                         entry
-                        if i == 0 or i == len(excinfo.traceback) - 1
+                        if i == 0 or i == len(ntraceback) - 1
                         else entry.with_repr_style("short")
-                        for i, entry in enumerate(excinfo.traceback)
+                        for i, entry in enumerate(ntraceback)
                     )
 
+            return ntraceback
+        return excinfo.traceback
+
     # TODO: Type ignored -- breaks Liskov Substitution.
     def repr_failure(  # type: ignore[override]
         self,

src/_pytest/unittest.py

@@ -334,15 +334,16 @@ class TestCaseFunction(Function):
             finally:
                 delattr(self._testcase, self.name)
 
-    def _prunetraceback(
+    def _traceback_filter(
         self, excinfo: _pytest._code.ExceptionInfo[BaseException]
-    ) -> None:
-        super()._prunetraceback(excinfo)
-        traceback = excinfo.traceback.filter(
+    ) -> _pytest._code.Traceback:
+        traceback = super()._traceback_filter(excinfo)
+        ntraceback = traceback.filter(
             lambda x: not x.frame.f_globals.get("__unittest"),
         )
-        if traceback:
-            excinfo.traceback = traceback
+        if not ntraceback:
+            ntraceback = traceback
+        return ntraceback
 
 
 @hookimpl(tryfirst=True)