Take care of some edge cases in the backtracker

sudoku/solvers/backtracker.py

@@ -12,21 +12,21 @@ def solve(sudoku):
     return _solve_square(sudoku, 0, 0)
 
 def _solve_square(sudoku, x, y):
+    should_backtrack = False
     for value in sudoku.possible_values:
+        should_backtrack = False
         try:
-            print('({},{}) trying {}'.format(x, y, value))
             sudoku.set(x, y, value)
         except SquareIsClue:
             # Do nothing with this square; continue on to the next square below.
-            print('({},{}) square is clue'.format(x,y))
             pass
         except ValueExistsInPeers:
-            print('({},{}) value exists in peer set'.format(x,y))
             # Try next value.
+            should_backtrack = True
             continue
         except NoPossibleValues:
             # Need to backtrack.
-            print('({},{}) backtracking'.format(x,y))
+            sudoku.unset(x, y)
             raise Backtrack()
 
         next_coord = _next_coord(sudoku, x, y)
@@ -34,9 +34,19 @@ def _solve_square(sudoku, x, y):
             break
 
         try:
-            _solve_square(sudoku, *next_coord)
+            return _solve_square(sudoku, *next_coord)
         except Backtrack:
+            should_backtrack = True
             continue
+
+    if should_backtrack:
+        # Unhandled backtrack. Pop out of this one too.
+        try:
+            sudoku.unset(x, y)
+        except SquareIsClue:
+            pass
+        raise Backtrack()
+
     return sudoku
 
 def _next_coord(sudoku, x, y):