Prevent approx from being used without a comparison (#9061)

Some of the top search-engine hits for pytest.approx use the function without actually comparing it to anything.

This PR will cause these tests to fail by implementing approx.__bool__() to raise an AssertionError that briefly explains how to correctly use approx.

Author: kalekundert

changelog/9061.breaking.rst

@@ -0,0 +1,15 @@
+Using :func:`pytest.approx` in a boolean context now raises an error hinting at the proper usage.
+
+It is apparently common for users to mistakenly use ``pytest.approx`` like this:
+
+.. code-block:: python
+
+    assert pytest.approx(actual, expected)
+
+While the correct usage is:
+
+.. code-block:: python
+
+    assert actual == pytest.approx(expected)
+
+The new error message helps catch those mistakes.

src/_pytest/python_api.py

@@ -100,6 +100,11 @@ def __eq__(self, actual) -> bool:
             a == self._approx_scalar(x) for a, x in self._yield_comparisons(actual)
         )
 
+    def __bool__(self):
+        raise AssertionError(
+            "approx() is not supported in a boolean context.\nDid you mean: `assert a == approx(b)`?"
+        )
+
     # Ignore type because of https://github.com/python/mypy/issues/4266.
     __hash__ = None  # type: ignore
 

testing/python/approx.py

@@ -319,6 +319,12 @@ def test_repr_nd_array(self, value, expected_repr_string):
         np_array = np.array(value)
         assert repr(approx(np_array)) == expected_repr_string
 
+    def test_bool(self):
+        with pytest.raises(AssertionError) as err:
+            assert approx(1)
+
+        assert err.match(r"approx\(\) is not supported in a boolean context")
+
     def test_operator_overloading(self):
         assert 1 == approx(1, rel=1e-6, abs=1e-12)
         assert not (1 != approx(1, rel=1e-6, abs=1e-12))