From af42e7154a39efac07668c9db238c90c31b4c1e0 Mon Sep 17 00:00:00 2001
From: Kale Kundert <kale@thekunderts.net>
Date: Mon, 30 Aug 2021 14:19:31 -0400
Subject: [PATCH] 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.
---
 changelog/9061.breaking.rst | 15 +++++++++++++++
 src/_pytest/python_api.py   |  5 +++++
 testing/python/approx.py    |  6 ++++++
 3 files changed, 26 insertions(+)
 create mode 100644 changelog/9061.breaking.rst

diff --git a/changelog/9061.breaking.rst b/changelog/9061.breaking.rst
new file mode 100644
index 000000000..bf639e132
--- /dev/null
+++ b/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.
diff --git a/src/_pytest/python_api.py b/src/_pytest/python_api.py
index 847991a37..8332bbe0a 100644
--- a/src/_pytest/python_api.py
+++ b/src/_pytest/python_api.py
@@ -100,6 +100,11 @@ class ApproxBase:
             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
 
diff --git a/testing/python/approx.py b/testing/python/approx.py
index d4a152f3e..0d411d8a6 100644
--- a/testing/python/approx.py
+++ b/testing/python/approx.py
@@ -319,6 +319,12 @@ class TestApprox:
         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))