Fix #741: ValueError in WordSwapChangeNumber._alter_number for negative numbers

- Ensure change is always positive: change = abs(int(num * max_change)) + 1
- Add range validation before calling np.random.randint()
- Return empty list for invalid ranges instead of raising exception
- Maintain backward compatibility for all valid inputs

Fixes #741

Author: OpenClaw Assistant

textattack/transformations/word_swaps/word_swap_change_number.py

@@ -104,11 +104,29 @@ def _alter_number(self, num):
         """Helper function of _get_new_number, replace a number with another
         random number within the range of self.max_change."""
         if num not in [0, 2, 4]:
-            change = int(num * self.max_change) + 1
+            # Ensure change is always positive to avoid invalid ranges
+            change = abs(int(num * self.max_change)) + 1
+            
             if num >= 0:
-                num_list = np.random.randint(max(num - change, 1), num + change, self.n)
+                # For positive numbers, ensure we don't go below 1
+                low = max(num - change, 1)
+                high = num + change
+                # Ensure valid range for np.random.randint
+                if low < high:
+                    num_list = np.random.randint(low, high, self.n)
+                else:
+                    # If range is invalid, return empty list
+                    return []
             else:
-                num_list = np.random.randint(num - change, min(0, num + change), self.n)
+                # For negative numbers
+                low = num - change
+                high = min(0, num + change)
+                # Ensure valid range for np.random.randint
+                if low < high:
+                    num_list = np.random.randint(low, high, self.n)
+                else:
+                    # If range is invalid, return empty list
+                    return []
             return num_list
         return []