Attempt #1 at a backtracking solver

sudoku/solvers/__init__.py

@@ -1,8 +1,6 @@
 # Eryn Wells <eryn@erynwells.me>
 '''
+A library of Sudoku solvers.
 '''
 
-class Solver:
-    def solve(self, sudoku):
-        # TODO: Any subclass of Solver should implement this.
-        raise NotImplementedError
+from . import backtracker

sudoku/solvers/backtracker.py

@@ -0,0 +1,48 @@
+# Eryn Wells <eryn@erynwells.me>
+
+from .. import SquareIsClue, ValueExistsInPeers, NoPossibleValues
+
+class Backtrack(Exception):
+    pass
+
+def solve(sudoku):
+    '''
+    Implements a recursive backtracking Sudoku solver.
+    '''
+    return _solve_square(sudoku, 0, 0)
+
+def _solve_square(sudoku, x, y):
+    for value in sudoku.possible_values:
+        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.
+            continue
+        except NoPossibleValues:
+            # Need to backtrack.
+            print('({},{}) backtracking'.format(x,y))
+            raise Backtrack()
+
+        next_coord = _next_coord(sudoku, x, y)
+        if not next_coord:
+            break
+
+        try:
+            _solve_square(sudoku, *next_coord)
+        except Backtrack:
+            continue
+    return sudoku
+
+def _next_coord(sudoku, x, y):
+    x += 1
+    if x >= sudoku.row_size:
+        (x, y) = (0, y + 1)
+    if y >= sudoku.row_size:
+        return None
+    return (x, y)