BUG: NumericalExpressions fail to merge with too many inputs

tests/pipeline/test_numerical_expression.py

@@ -8,7 +8,9 @@ from operator import (
     methodcaller,
     mul,
     ne,
+    sub,
 )
+from string import ascii_uppercase
 from unittest import TestCase
 
 import numpy
@@ -147,6 +149,41 @@ class NumericalExpressionTestCase(TestCase):
         with self.assertRaises(TypeError):
             (f > f) > f
 
+    def test_many_inputs(self):
+        """
+        Test adding NumericalExpressions with >10 inputs.
+        """
+        # Create an initial NumericalExpression by adding two factors together.
+        f = self.f
+        expr = f + f
+
+        self.fake_raw_data = {f: full((5, 5), 0, float)}
+        expected = 0
+
+        # Alternate between adding and subtracting factors. Because subtraction
+        # is not commutative, this ensures that we are combining factors in the
+        # correct order.
+        ops = (add, sub)
+
+        for i, name in enumerate(ascii_uppercase):
+            op = ops[i % 2]
+            NewFactor = type(
+                name,
+                (Factor,),
+                dict(dtype=float64_dtype, inputs=(), window_length=0),
+            )
+            new_factor = NewFactor()
+
+            # Again we need a NumericalExpression, so add two factors together.
+            new_expr = new_factor + new_factor
+            self.fake_raw_data[new_factor] = full((5, 5), i + 1, float)
+            expr = op(expr, new_expr)
+
+            # Double the expected output since each factor is counted twice.
+            expected = op(expected, (i + 1) * 2)
+
+        self.check_output(expr, full((5, 5), expected, float))
+
     def test_combine_datetimes(self):
         with self.assertRaises(TypeError) as e:
             self.d + self.d