feat: actionable error when a memray Tracker is already active (pytest-memray interop)

README.md

@@ -175,8 +175,9 @@ directions:
 They coexist in one suite. Reach for **pytest-memray** to assert a ceiling, catch
 a leak, or see which function allocated; reach for **pytest-benchmem** to track a
 measured number over time and gate on a delta. One caveat: memray won't nest two
-trackers, so don't run `pytest --memray` and a benchmem fixture on the *same*
-test.
+trackers, so don't run `pytest --memray` and a benchmem fixture on the *same* test
+— if you do, benchmem fails that test with an actionable error rather than a terse
+memray one.
 
 ## Install
 

src/pytest_benchmem/memray.py

@@ -145,15 +145,38 @@ def _compute_statistics() -> Callable[[str], Any]:
     return compute_statistics
 
 
+#: Substring of memray's error when a second ``Tracker`` is started while one is active.
+#: memray raises a bare ``RuntimeError("No more than one Tracker instance can be active
+#: at the same time")``; we match on this to turn it into an actionable message.
+_NESTED_TRACKER_MARKER = "more than one Tracker"
+
+
 def _track_once(action: Action) -> Measurement:
-    """One fresh tracker run → a :class:`Measurement` via memray stats."""
+    """One fresh tracker run → a :class:`Measurement` via memray stats.
+
+    memray allows only one active ``Tracker`` per process, so if one is already
+    running — most often pytest-memray's ``--memray`` profiling the same test — the
+    nested ``Tracker`` raises. We translate that into an actionable error rather than
+    surfacing memray's terse one.
+    """
     import memray
 
     compute_statistics = _compute_statistics()
     with tempfile.TemporaryDirectory(prefix="pytest-benchmem-") as tmp:
         out = Path(tmp) / "track.bin"  # memray must create the file itself
-        with memray.Tracker(out):
-            action()
+        try:
+            with memray.Tracker(out):
+                action()
+        except RuntimeError as exc:
+            if _NESTED_TRACKER_MARKER in str(exc):
+                raise RuntimeError(
+                    "pytest-benchmem couldn't start its memray pass because another memray "
+                    "Tracker is already active — most likely pytest-memray's `--memray` running "
+                    "on the same test. memray allows only one Tracker per process. Run the two "
+                    "on different tests (or different sessions): pytest-memray for whole-test "
+                    "limits/leaks, pytest-benchmem for the benchmarked action's memory."
+                ) from exc
+            raise
         s = compute_statistics(str(out))
         return Measurement(
             peak_bytes=int(s.peak_memory_allocated),

tests/test_memray.py

@@ -21,6 +21,19 @@ def test_measure_peak_returns_positive_bytes():
     assert peak > 1_000_000  # a list of 1e6 ints is several MB; bytes, not MiB
 
 
+def test_nested_tracker_raises_actionable_error(tmp_path):
+    """If a memray Tracker is already active (e.g. pytest-memray's --memray on the same
+    test), benchmem's pass re-raises memray's terse error as an actionable one."""
+    import memray
+
+    # outer Tracker stands in for pytest-memray's --memray; it must be active first
+    with (
+        memray.Tracker(tmp_path / "outer.bin"),
+        pytest.raises(RuntimeError, match="another memray Tracker is already active"),
+    ):
+        measure_memory(lambda: [0] * 1000)
+
+
 def test_repeats_takes_min_of_n():
     # min-of-N: a smaller allocation never reports more than a larger one would.
     small = measure_peak(lambda: [0] * 100_000, repeats=3)