code: handle repr'ing empty tracebacks gracefully

By "empty traceback" I mean a traceback all of whose entries have been
filtered/cut/pruned out.

Currently, if an empty traceback needs to be repr'ed, the last entry
before the filtering is used instead (added in
accd962).

Showing a hidden frame is not so good IMO. This commit does the
following instead:

1. Shows details of the exception.
2. Shows a message about how the full trace can be seen.

Example:

```
_____________ test _____________

E   ZeroDivisionError: division by zero
All traceback entries are hidden. Pass `--full-trace` to see hidden and internal frames.
```

Also handles `--tb=native`, though there the `--full-trace` bit is not
shown.

This commit contains some pieces from
431ec6d (which has been reverted).

Helps towards fixing issue # 1904.

Co-authored-by: Felix Hofstätter <[email protected]>

Author: bluetech

AUTHORS

@@ -128,6 +128,7 @@ Erik M. Bray
 Evan Kepner
 Fabien Zarifian
 Fabio Zadrozny
+Felix Hofstätter
 Felix Nieuwenhuizen
 Feng Ma
 Florian Bruhin

src/_pytest/_code/code.py

@@ -411,13 +411,13 @@ def filter(
         """
         return Traceback(filter(fn, self), self._excinfo)
 
-    def getcrashentry(self) -> TracebackEntry:
+    def getcrashentry(self) -> Optional[TracebackEntry]:
         """Return last non-hidden traceback entry that lead to the exception of a traceback."""
         for i in range(-1, -len(self) - 1, -1):
             entry = self[i]
             if not entry.ishidden():
                 return entry
-        return self[-1]
+        return None
 
     def recursionindex(self) -> Optional[int]:
         """Return the index of the frame/TracebackEntry where recursion originates if
@@ -598,9 +598,11 @@ def errisinstance(
         """
         return isinstance(self.value, exc)
 
-    def _getreprcrash(self) -> "ReprFileLocation":
+    def _getreprcrash(self) -> Optional["ReprFileLocation"]:
         exconly = self.exconly(tryshort=True)
         entry = self.traceback.getcrashentry()
+        if entry is None:
+            return None
         path, lineno = entry.frame.code.raw.co_filename, entry.lineno
         return ReprFileLocation(path, lineno + 1, exconly)
 
@@ -647,7 +649,9 @@ def getrepr(
             return ReprExceptionInfo(
                 reprtraceback=ReprTracebackNative(
                     traceback.format_exception(
-                        self.type, self.value, self.traceback[0]._rawentry
+                        self.type,
+                        self.value,
+                        self.traceback[0]._rawentry if self.traceback else None,
                     )
                 ),
                 reprcrash=self._getreprcrash(),
@@ -803,12 +807,16 @@ def repr_locals(self, locals: Mapping[str, object]) -> Optional["ReprLocals"]:
 
     def repr_traceback_entry(
         self,
-        entry: TracebackEntry,
+        entry: Optional[TracebackEntry],
         excinfo: Optional[ExceptionInfo[BaseException]] = None,
     ) -> "ReprEntry":
         lines: List[str] = []
-        style = entry._repr_style if entry._repr_style is not None else self.style
-        if style in ("short", "long"):
+        style = (
+            entry._repr_style
+            if entry is not None and entry._repr_style is not None
+            else self.style
+        )
+        if style in ("short", "long") and entry is not None:
             source = self._getentrysource(entry)
             if source is None:
                 source = Source("???")
@@ -857,17 +865,21 @@ def repr_traceback(self, excinfo: ExceptionInfo[BaseException]) -> "ReprTracebac
         else:
             extraline = None
 
+        if not traceback:
+            if extraline is None:
+                extraline = "All traceback entries are hidden. Pass `--full-trace` to see hidden and internal frames."
+            entries = [self.repr_traceback_entry(None, excinfo)]
+            return ReprTraceback(entries, extraline, style=self.style)
+
         last = traceback[-1]
-        entries = []
         if self.style == "value":
-            reprentry = self.repr_traceback_entry(last, excinfo)
-            entries.append(reprentry)
+            entries = [self.repr_traceback_entry(last, excinfo)]
             return ReprTraceback(entries, None, style=self.style)
 
-        for index, entry in enumerate(traceback):
-            einfo = (last == entry) and excinfo or None
-            reprentry = self.repr_traceback_entry(entry, einfo)
-            entries.append(reprentry)
+        entries = [
+            self.repr_traceback_entry(entry, excinfo if last == entry else None)
+            for entry in traceback
+        ]
         return ReprTraceback(entries, extraline, style=self.style)
 
     def _truncate_recursive_traceback(
@@ -924,6 +936,7 @@ def repr_excinfo(
         seen: Set[int] = set()
         while e is not None and id(e) not in seen:
             seen.add(id(e))
+
             if excinfo_:
                 # Fall back to native traceback as a temporary workaround until
                 # full support for exception groups added to ExceptionInfo.
@@ -950,8 +963,8 @@ def repr_excinfo(
                     traceback.format_exception(type(e), e, None)
                 )
                 reprcrash = None
-
             repr_chain += [(reprtraceback, reprcrash, descr)]
+
             if e.__cause__ is not None and self.chain:
                 e = e.__cause__
                 excinfo_ = (
@@ -1042,7 +1055,7 @@ def toterminal(self, tw: TerminalWriter) -> None:
 @dataclasses.dataclass(eq=False)
 class ReprExceptionInfo(ExceptionRepr):
     reprtraceback: "ReprTraceback"
-    reprcrash: "ReprFileLocation"
+    reprcrash: Optional["ReprFileLocation"]
 
     def toterminal(self, tw: TerminalWriter) -> None:
         self.reprtraceback.toterminal(tw)
@@ -1147,8 +1160,8 @@ def _write_entry_lines(self, tw: TerminalWriter) -> None:
 
     def toterminal(self, tw: TerminalWriter) -> None:
         if self.style == "short":
-            assert self.reprfileloc is not None
-            self.reprfileloc.toterminal(tw)
+            if self.reprfileloc:
+                self.reprfileloc.toterminal(tw)
             self._write_entry_lines(tw)
             if self.reprlocals:
                 self.reprlocals.toterminal(tw, indent=" " * 8)

src/_pytest/nodes.py

@@ -452,10 +452,7 @@ def _repr_failure_py(
         if self.config.getoption("fulltrace", False):
             style = "long"
         else:
-            tb = _pytest._code.Traceback([excinfo.traceback[-1]])
             self._prunetraceback(excinfo)
-            if len(excinfo.traceback) == 0:
-                excinfo.traceback = tb
             if style == "auto":
                 style = "long"
         # XXX should excinfo.getrepr record all data and toterminal() process it?

src/_pytest/reports.py

@@ -347,6 +347,10 @@ def from_item_and_call(cls, item: Item, call: "CallInfo[None]") -> "TestReport":
             elif isinstance(excinfo.value, skip.Exception):
                 outcome = "skipped"
                 r = excinfo._getreprcrash()
+                if r is None:
+                    raise ValueError(
+                        "There should always be a traceback entry for skipping a test."
+                    )
                 if excinfo.value._use_item_location:
                     path, line = item.reportinfo()[:2]
                     assert line is not None

testing/code/test_excinfo.py

@@ -294,6 +294,7 @@ def f():
         excinfo = pytest.raises(ValueError, f)
         tb = excinfo.traceback
         entry = tb.getcrashentry()
+        assert entry is not None
         co = _pytest._code.Code.from_function(h)
         assert entry.frame.code.path == co.path
         assert entry.lineno == co.firstlineno + 1
@@ -309,12 +310,7 @@ def f():
             g()
 
         excinfo = pytest.raises(ValueError, f)
-        tb = excinfo.traceback
-        entry = tb.getcrashentry()
-        co = _pytest._code.Code.from_function(g)
-        assert entry.frame.code.path == co.path
-        assert entry.lineno == co.firstlineno + 2
-        assert entry.frame.code.name == "g"
+        assert excinfo.traceback.getcrashentry() is None
 
 
 def test_excinfo_exconly():
@@ -1577,18 +1573,19 @@ def test_exceptiongroup(pytester: Pytester, outer_chain, inner_chain) -> None:
     _exceptiongroup_common(pytester, outer_chain, inner_chain, native=False)
 
 
-def test_all_entries_hidden_doesnt_crash(pytester: Pytester) -> None:
-    """Regression test for #10903.
-
-    We're not really sure what should be *displayed* here, so this test
-    just verified that at least it doesn't crash.
-    """
+@pytest.mark.parametrize("tbstyle", ("long", "short", "auto", "line", "native"))
+def test_all_entries_hidden(pytester: Pytester, tbstyle: str) -> None:
+    """Regression test for #10903."""
     pytester.makepyfile(
         """
         def test():
             __tracebackhide__ = True
             1 / 0
     """
     )
-    result = pytester.runpytest()
+    result = pytester.runpytest("--tb", tbstyle)
     assert result.ret == 1
+    if tbstyle != "line":
+        result.stdout.fnmatch_lines(["*ZeroDivisionError: division by zero"])
+    if tbstyle not in ("line", "native"):
+        result.stdout.fnmatch_lines(["All traceback entries are hidden.*"])