Goban._valid_move() should allow placing a stone into a liberty-less space if it will result in a capture.

pygo.py

@@ -154,11 +154,29 @@ class Goban:
     # if it will result in a capture...
     def _valid_move(self, pos):
         x, y = pos
-        liberties = self._has_liberties((x, y+1), self.turn, 'r')
-        liberties += self._has_liberties((x-1, y), self.turn, 'u')
-        liberties += self._has_liberties((x, y-1), self.turn, 'l')
-        liberties += self._has_liberties((x+1, y), self.turn, 'd')
-        return liberties
+        if x < 0 or x > 18 or y < 0 or y > 18:
+            return False
+
+        # Can't play atop another stone
+        if self.board[x][y].state != -1:
+            return False
+
+        # Temporarily place the stone
+        self.board[x][y].state = self.turn
+
+        liberties = self._has_liberties(pos, self.turn)
+
+        opponent = (self.turn + 1) % 2
+
+        kills_group = False
+        for d in [(x, y+1), (x, y-1), (x+1, y), (x-1, y)]:
+            if not self._has_liberties(d, opponent):
+                kills_group = True
+                break
+
+        self.board[x][y].state = -1
+
+        return liberties > 0 or kills_group
 
 
     # Recursively find whether there are liberties for the group