handle comparison results which raise when asked for their truth value

_pytest/assertrewrite.py

@@ -25,7 +25,11 @@ def _format_boolop(operands, explanations, is_or):
 
 def _call_reprcompare(ops, results, expls, each_obj):
     for i, res, expl in zip(range(len(ops)), results, expls):
-        if not res:
+        try:
+            done = not res
+        except Exception:
+            done = True
+        if done:
             break
     if py.code._reprcompare is not None:
         custom = py.code._reprcompare(ops[i], each_obj[i], each_obj[i + 1])

testing/test_assertrewrite.py

@@ -239,3 +239,17 @@ class TestAssertionRewrite:
         def f():
             assert 1 < 3 < 5 <= 4 < 7
         assert getmsg(f) == "assert 5 <= 4"
+
+    def test_assert_raising_nonzero_in_comparison(self):
+        def f():
+            class A(object):
+                def __nonzero__(self):
+                    raise ValueError(42)
+                def __lt__(self, other):
+                    return A()
+                def __repr__(self):
+                    return "<MY42 object>"
+            def myany(x):
+                return False
+            assert myany(A() < 0)
+        assert "<MY42 object> < 0" in getmsg(f)