Allow timedelta rel tolerance in approx

Co-authored-by: Codex <codex@openai.com>

Author: hamza-mobeen

src/_pytest/python_api.py

@@ -565,34 +565,40 @@ class ApproxTimedelta(ApproxBase):
     """Perform approximate comparisons where the expected value is a
     datetime or timedelta.
 
-    Requires an explicit absolute tolerance as a timedelta.
-    Relative tolerance is not supported for time-based comparisons.
+    Requires an explicit tolerance as a timedelta.
+    Relative tolerance is not supported for datetime comparisons.
     """
 
     def __init__(self, expected, rel=None, abs=None, nan_ok: bool = False) -> None:
         __tracebackhide__ = True
-        if rel is not None:
+        if isinstance(expected, datetime) and rel is not None:
             raise TypeError(
                 "pytest.approx() does not support relative tolerance for "
-                "datetime/timedelta comparisons. Use abs=timedelta(...) instead."
+                "datetime comparisons. Use abs=timedelta(...) instead."
             )
         if nan_ok:
             raise TypeError(
                 "pytest.approx() does not support nan_ok for "
                 "datetime/timedelta comparisons."
             )
-        if abs is None:
+        if abs is None and rel is None:
             raise TypeError(
-                "pytest.approx() requires an absolute tolerance for "
+                "pytest.approx() requires an explicit tolerance for "
                 "datetime/timedelta comparisons: "
                 "e.g. approx(expected, abs=timedelta(seconds=1))"
             )
-        if not isinstance(abs, timedelta):
+        if abs is not None and not isinstance(abs, timedelta):
             raise TypeError(
                 f"absolute tolerance for datetime/timedelta must be a "
                 f"timedelta, got {type(abs).__name__}"
             )
-        super().__init__(expected, rel=None, abs=abs, nan_ok=False)
+        if rel is not None and not isinstance(rel, timedelta):
+            raise TypeError(
+                f"relative tolerance for timedelta must be a "
+                f"timedelta, got {type(rel).__name__}"
+            )
+        tolerance = max(t for t in (abs, rel) if t is not None)
+        super().__init__(expected, rel=None, abs=tolerance, nan_ok=False)
 
     def __repr__(self) -> str:
         return f"{self.expected} ± {self.abs}"

testing/python/approx.py

@@ -1167,25 +1167,48 @@ def test_timedelta_outside_tolerance(self):
         td2 = timedelta(seconds=102)
         assert td1 != approx(td2, abs=timedelta(seconds=1))
 
-    def test_requires_abs(self):
+    def test_timedelta_rel_within_tolerance(self):
+        from datetime import timedelta
+
+        td1 = timedelta(seconds=100)
+        td2 = timedelta(seconds=100.5)
+        assert td1 == approx(td2, rel=timedelta(seconds=1))
+
+    def test_timedelta_rel_outside_tolerance(self):
+        from datetime import timedelta
+
+        td1 = timedelta(seconds=100)
+        td2 = timedelta(seconds=102)
+        assert td1 != approx(td2, rel=timedelta(seconds=1))
+
+    def test_requires_tolerance(self):
         from datetime import datetime
 
-        with pytest.raises(TypeError, match="requires an absolute tolerance"):
+        with pytest.raises(TypeError, match="requires an explicit tolerance"):
             approx(datetime(2024, 1, 1))
 
-    def test_rejects_rel(self):
+    def test_datetime_rejects_rel(self):
         from datetime import datetime
         from datetime import timedelta
 
         with pytest.raises(TypeError, match="does not support relative tolerance"):
             approx(datetime(2024, 1, 1), rel=0.1, abs=timedelta(seconds=1))
 
+        with pytest.raises(TypeError, match="does not support relative tolerance"):
+            approx(datetime(2024, 1, 1), rel=timedelta(seconds=1))
+
     def test_abs_must_be_timedelta(self):
         from datetime import datetime
 
         with pytest.raises(TypeError, match="must be a timedelta"):
             approx(datetime(2024, 1, 1), abs=1.0)
 
+    def test_timedelta_rel_must_be_timedelta(self):
+        from datetime import timedelta
+
+        with pytest.raises(TypeError, match="must be a timedelta"):
+            approx(timedelta(seconds=1), rel=0.1)
+
     def test_rejects_nan_ok(self):
         from datetime import datetime
         from datetime import timedelta