Disallow unordered sequences in pytest.approx

Fix pytest-dev#9692

changelog/9692.improvement.rst

@@ -0,0 +1 @@
+:func:`pytest.approx` now raises a :class:`TypeError` when given an unordered sequence (such as :class:`set`).

src/_pytest/python_api.py

@@ -311,7 +311,7 @@ class ApproxSequencelike(ApproxBase):
 
     def __repr__(self) -> str:
         seq_type = type(self.expected)
-        if seq_type not in (tuple, list, set):
+        if seq_type not in (tuple, list):
             seq_type = list
         return "approx({!r})".format(
             seq_type(self._approx_scalar(x) for x in self.expected)
@@ -372,6 +372,9 @@ def _yield_comparisons(self, actual):
 
     def _check_type(self) -> None:
         __tracebackhide__ = True
+        if not isinstance(self.expected, Sequence):
+            msg = f"pytest.approx() only supports ordered sequences, but got: {repr(self.expected)}"
+            raise TypeError(msg)
         for index, x in enumerate(self.expected):
             if isinstance(x, type(self.expected)):
                 msg = "pytest.approx() does not support nested data structures: {!r} at index {}\n  full sequence: {}"
@@ -515,7 +518,7 @@ class ApproxDecimal(ApproxScalar):
 
 
 def approx(expected, rel=None, abs=None, nan_ok: bool = False) -> ApproxBase:
-    """Assert that two numbers (or two sets of numbers) are equal to each other
+    """Assert that two numbers (or two ordered sequences of numbers) are equal to each other
     within some tolerance.
 
     Due to the :std:doc:`tutorial/floatingpoint`, numbers that we

testing/python/approx.py

@@ -868,3 +868,8 @@ def __len__(self):
 
         expected = MySizedIterable()
         assert [1, 2, 3, 4] == approx(expected)
+
+    def test_allow_ordered_sequences_only(self) -> None:
+        """pytest.approx() should raise an error on unordered sequences (#9692)."""
+        with pytest.raises(TypeError, match="only supports ordered sequences"):
+            assert {1, 2, 3} == approx({1, 2, 3})