Disallow unordered sequences in pytest.approx

Fix pytest-dev#9692

changelog/9692.improvement.rst

@@ -0,0 +1,3 @@
+:func:`pytest.approx` now raises a :class:`TypeError` when given an unordered sequence (such as :class:`set`).
+
+Note that this implies that custom classes which only implement ``__iter__`` and ``__len__`` are no longer supported as they don't guarantee order.

src/_pytest/python_api.py

@@ -1,5 +1,6 @@
 import math
 import pprint
+from collections.abc import Collection
 from collections.abc import Sized
 from decimal import Decimal
 from numbers import Complex
@@ -8,7 +9,6 @@
 from typing import Callable
 from typing import cast
 from typing import Generic
-from typing import Iterable
 from typing import List
 from typing import Mapping
 from typing import Optional
@@ -306,12 +306,12 @@ def _check_type(self) -> None:
                 raise TypeError(msg.format(key, value, pprint.pformat(self.expected)))
 
 
-class ApproxSequencelike(ApproxBase):
+class ApproxSequenceLike(ApproxBase):
     """Perform approximate comparisons where the expected value is a sequence of numbers."""
 
     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)
@@ -515,7 +515,7 @@ 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
@@ -547,16 +547,11 @@ def approx(expected, rel=None, abs=None, nan_ok: bool = False) -> ApproxBase:
         >>> 0.1 + 0.2 == approx(0.3)
         True
 
-    The same syntax also works for sequences of numbers::
+    The same syntax also works for ordered sequences of numbers::
 
         >>> (0.1 + 0.2, 0.2 + 0.4) == approx((0.3, 0.6))
         True
 
-    Dictionary *values*::
-
-        >>> {'a': 0.1 + 0.2, 'b': 0.2 + 0.4} == approx({'a': 0.3, 'b': 0.6})
-        True
-
     ``numpy`` arrays::
 
         >>> import numpy as np                                                          # doctest: +SKIP
@@ -569,6 +564,20 @@ def approx(expected, rel=None, abs=None, nan_ok: bool = False) -> ApproxBase:
         >>> np.array([0.1, 0.2]) + np.array([0.2, 0.1]) == approx(0.3) # doctest: +SKIP
         True
 
+    Only ordered sequences are supported, because ``approx`` needs
+    to infer the relative position of the sequences without ambiguity. This means
+    ``sets`` and other unordered sequences are not supported.
+
+    Finally, dictionary *values* can also be compared::
+
+        >>> {'a': 0.1 + 0.2, 'b': 0.2 + 0.4} == approx({'a': 0.3, 'b': 0.6})
+        True
+
+    The comparision will be true if both mappings have the same keys and their
+    respective values match the expected tolerances.
+
+    **Tolerances**
+
     By default, ``approx`` considers numbers within a relative tolerance of
     ``1e-6`` (i.e. one part in a million) of its expected value to be equal.
     This treatment would lead to surprising results if the expected value was
@@ -708,12 +717,19 @@ def approx(expected, rel=None, abs=None, nan_ok: bool = False) -> ApproxBase:
         expected = _as_numpy_array(expected)
         cls = ApproxNumpy
     elif (
-        isinstance(expected, Iterable)
+        hasattr(expected, "__getitem__")
         and isinstance(expected, Sized)
         # Type ignored because the error is wrong -- not unreachable.
         and not isinstance(expected, STRING_TYPES)  # type: ignore[unreachable]
     ):
-        cls = ApproxSequencelike
+        cls = ApproxSequenceLike
+    elif (
+        isinstance(expected, Collection)
+        # Type ignored because the error is wrong -- not unreachable.
+        and not isinstance(expected, STRING_TYPES)  # type: ignore[unreachable]
+    ):
+        msg = f"pytest.approx() only supports ordered sequences, but got: {repr(expected)}"
+        raise TypeError(msg)
     else:
         cls = ApproxScalar
 

testing/python/approx.py

@@ -858,13 +858,18 @@ def test_numpy_scalar_with_array(self):
         assert approx(expected, rel=5e-7, abs=0) == actual
         assert approx(expected, rel=5e-8, abs=0) != actual
 
-    def test_generic_sized_iterable_object(self):
-        class MySizedIterable:
-            def __iter__(self):
-                return iter([1, 2, 3, 4])
+    def test_generic_ordered_sequence(self):
+        class MySequence:
+            def __getitem__(self, i):
+                return [1, 2, 3, 4][i]
 
             def __len__(self):
                 return 4
 
-        expected = MySizedIterable()
+        expected = MySequence()
         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})