Add detail to traced values where str() is ambiguous

[RonnyPfannschmidt]

AI-authored. I asked Claude Code (Opus 5) to work out a rendering for traced
values that keeps pytest's trace readable while surfacing the details str()
drops. The code, the tests, the measurements and the text below are the agent's
work. I read it and I am posting it, and I will follow up on review comments
myself.

Stacked on #728 — that must land first. Until it does, the diff here includes its
two commits; the change proposed by this PR is the last commit,
Add detail to traced values where str() is ambiguous.

Why

#728 fixes the two tracing crashes without touching the output, and says explicitly that
the type-visibility idea from #627/#681 should be argued on its own. This is that
argument.

Rendering every value with repr(), as #681 proposed, changes 216 of 1329 lines in a
real pytest --debug run, and the overwhelming majority of that is quotes added around
strings that were already readable. But str() does drop things a reader needs:

in the trace today	the problem
left: / right:	an empty string is indistinguishable from no value at all — in a comparison trace
val: with space	no visible boundaries; leading and trailing space are invisible
exitstatus: 1	an IntEnum prints as a bare number, the member name is lost
collection_path: /x path: /x	a PosixPath and a py.path.local pointing at the same place look identical
a multi-line value	runs out to column 0 and reads as a trace line of its own

The rule

• a string that reads unambiguously as itself — non-empty, printable, no spaces — stays bare
• a string that is empty, or carries whitespace, gets repr()
• an enum.Enum or an os.PathLike gets repr()
• a multi-line value is drawn as a block
• everything else keeps str(), unchanged

What it costs

Measured on a real pytest --debug run: 45 of 1329 lines change (3.4%), against 216
for blanket repr(). By category: 21 paths, 11 whitespace strings, 3 empty strings, 2
enums, 2 blocks.

  exitstatus: 1                      →  exitstatus: <ExitCode.TESTS_FAILED: 1>
  left:                              →  left: ''
  right:                             →  right: ''
  collection_path: DIR/.benchmarks   →  collection_path: PosixPath('DIR/.benchmarks')
  path: DIR/.benchmarks              →  path: local('DIR/.benchmarks')
  val: with space                    →  val: 'with space'
  plugin_name: lfplugin              →  plugin_name: lfplugin        (unchanged)
  config: <_pytest.config.Config…>   →  config: <_pytest.config.Config…>   (unchanged)

Multi-line values

Today a multi-line value breaks the layout — the continuation escapes to column 0 and
reads as a top-level trace line:

            orig: isinstance(f, pathlib.Path)
            expl: True
 +  where True = isinstance(PosixPath('/tmp/pytest-N/test_path0'), <class 'pathlib.Path'>)
 +    where <class 'pathlib.Path'> = pathlib.Path
        finish pytest_assertion_pass --> [] [hook]

With this change the value is boxed, so its extent is visible at a glance:

            orig: 'isinstance(f, pathlib.Path)'
            expl:
              | True
              |  +  where True = isinstance(PosixPath('/tmp/pytest-N/test_path0'), <class 'pathlib.Path'>)
              \  +    where <class 'pathlib.Path'> = pathlib.Path
        finish pytest_assertion_pass --> [] [hook]

Rough edges, for review

• nodeid: test_mix.py::test_p[with space] gains quotes, because the parameter id
  contains a space. The rule fires correctly, but on something that is not really "a
  string with whitespace" in spirit.
• orig: n or n == "" becomes orig: 'n or n == ""' — nested quotes on source text.
• Lone surrogates now render '\ud800' rather than bare, since they are not
  isprintable(). Arguably clearer; it does change two tests added in Make hook tracing unable to fail a hook call #728.
• Rendering a traced value costs ~0.8 µs rather than ~0.2 µs, only when tracing is
  enabled — around 0.5 ms across a whole pytest run.

Testing

• uv run pytest — 187 passed.
• uv run pre-commit run -a — all hooks pass.
• _tracing.py at 100% statement and branch coverage.
• 8 new tests: 6 covering each branch of the rule, and 2 for the repr guards this change
  newly reaches — a path-like with a broken repr, and KeyboardInterrupt raised from a
  repr.
• pytest --debug output diffed against main: 45 changed lines, each one listed above.

🤖 Generated with Claude Code