Fix pandas-dev#16363: Prevent visit_BinOp from accessing value on UnaryOp

Author: alexcwatt

pandas/core/computation/expr.py

@@ -418,11 +418,11 @@ def _maybe_transform_eq_ne(self, node, left=None, right=None):
 
     def _maybe_downcast_constants(self, left, right):
         f32 = np.dtype(np.float32)
-        if left.is_scalar and not right.is_scalar and right.return_type == f32:
+        if left.is_scalar and hasattr(left, 'value') and not right.is_scalar and right.return_type == f32:
             # right is a float32 array, left is a scalar
             name = self.env.add_tmp(np.float32(left.value))
             left = self.term_type(name, self.env)
-        if right.is_scalar and not left.is_scalar and left.return_type == f32:
+        if right.is_scalar and hasattr(right, 'value') and not left.is_scalar and left.return_type == f32:
             # left is a float32 array, right is a scalar
             name = self.env.add_tmp(np.float32(right.value))
             right = self.term_type(name, self.env)

pandas/tests/computation/test_eval.py

@@ -608,6 +608,12 @@ def test_unary_in_array(self):
                       -False, False, ~False, +False,
                       -37, 37, ~37, +37], dtype=np.object_))
 
+    def test_float_comparison_bin_op(self):
+        # GH 16363
+        df = pd.DataFrame({'x': np.array([0], dtype=np.float32)})
+        res = df.eval('x < -0.1')
+        assert np.array_equal(res, np.array([False])), res
+
     def test_disallow_scalar_bool_ops(self):
         exprs = '1 or 2', '1 and 2'
         exprs += 'a and b', 'a or b'