[Bugfix] Fix permuation formatting with zero length separators

Author: elmar-hinz

challenges/challenge.py

@@ -380,10 +380,11 @@ def format_path(self, integers, backwards=False):
 
     def format_permutations(self, permutations, separator = '\n',
                             element_separator = ' '):
-        output = ''
+        entries = []
         for perm in permutations:
-            output += '('
-            output += element_separator.join(
+            entry = '('
+            entry += element_separator.join(
                 ('+' if i > 0 else '') + str(i) for i in perm)
-            output += ')' + separator
-        return output[:-len(separator)]
+            entry += ')'
+            entries.append(entry)
+        return separator.join(entries)

tests/test_challenge.py

@@ -286,7 +286,7 @@ def test_format_permuations(self):
         expect = '(+1 -2)\n(-3 +4)'
         self.assertEqual(expect, actual)
 
-    def test_format_permuations_with_user_defined_joint(self):
+    def test_format_permuations_with_user_defined_separator(self):
         """Show user defined separators can be used."""
         list = [[1, -2], [-3, 4]]
         actual = self.challenge.format_permutations(
@@ -296,3 +296,15 @@ def test_format_permuations_with_user_defined_joint(self):
         )
         expect = '(+1 | -2), (-3 | +4)'
         self.assertEqual(expect, actual)
+
+    def test_format_permuations_with_user_defined_separator_of_length_zero(
+            self):
+        """Show user defined separators can be used."""
+        list = [[1, -2], [-3, 4]]
+        actual = self.challenge.format_permutations(
+            list,
+            separator = '',
+            element_separator = ''
+        )
+        expect = '(+1-2)(-3+4)'
+        self.assertEqual(expect, actual)