Python: Recursion problems -

i trying make sudoku solver solves boards quickly. @ moment solver works on easy boards never terminates on harder boards. believe has recursion because easy boards not require recursion , hard boards do. appreciated.

import sys  def rowno(i):     return // 9  def colno(i):     return % 9  def boxno(i):     return (i // 9 // 3 )*3 + (i // 3) % 3  def isneighbor(i, j):     if rowno(i) == rowno(j) or colno(i) == colno(j) or boxno(i) == boxno(j):         return true     else:         return false  def getfilename():     if sys.platform == "win32":             filename = input("filename? ")     else:             filename = sys.argv[-1]     return filename   solutionlist = []  class board(object):     def __init__(self, puzzle):         self.puzzle = puzzle         self.board = [cell(int(value), idx) idx, value in     enumerate(puzzle)]         self.change = false      def printall(self):         print [cell.candidates cell in self.board]          #return str(" ")      def update(self):         self.change = false         l = [cell cell in self.board if len(cell.candidates) == 1]         in l:             j in xrange(81):                 if isneighbor(i.dex, j) , i.dex != j:                     old = self.board[j].candidates                     self.board[j].delcandidate(i.value)                     if len(old) != len(self.board[j].candidates):                         self.change = true      def tostring(self):         str1 = ''.join(str(e.value) e in self.board)         return str1       def solved(self):         cell in self.board:             if len(cell.candidates) != 1:                 return false         return true      def solve(self):         self.change = true         while self.change == true:             self.update()         if self.solved():             solutionlist.append(self.board)             return         l = [cell cell in self.board if len(cell.candidates) > 1]         in l:             j in i.candidates:                 newboard = board(self.tostring())                 curlen = 12                 curcell = -1                 u in l:                     if len(u.candidates)<curlen:                         curlen=len(u.candidates)                         curcell = u.dex                 c in newboard.board[curcell].candidates:                     newboard.board[curcell].candidates = [int(c)]                     newboard.board[curcell].value = int(c)                     newboard.solve()         return        def __repr__(self):         l = [cell.value cell in self.board]          return str(l)   class cell(object):     def __init__(self, value, dex):         self.value = value         self.dex = dex         if value == 0:             self.candidates = [1,2,3,4,5,6,7,8,9]         else:             self.candidates = [int(value)]      def __str__(self):         return str(self.value)      def delcandidate(self, value):         # deletes value candidate list         #return self.candidate.remove(value);         self.candidates = [x x in self.candidates if x != value]         if len(self.candidates) == 1:             self.value = self.candidates[0] easy =     "700583006006001405052006083300200958500078060648010300060802500003150072215600030" twosol = "000805200800000401705040009000100702040000000006430000030900000010006080000000000" hard = "040090008000000070060000120030020000005839060080600700050170600000043000003000200"  #easy solution: 794583216836721495152496783371264958529378164648915327967832541483159672215647839  b = board(hard) print b  b.solve() print "end of line" in solutionlist:     print [cell.value cell in i]     print "\n" 

one major issue line for in l: in solve method. since you're recursing, need fill in 1 cell - recursion take care of rest. instead of for in l:, recurse on 1 cell best candidate (curcell):

    l = [cell cell in self.board if len(cell.candidates) > 1]     if len(l) > 0:         newboard = board(self.tostring())         curlen = 12         curcell = -1         u in l:             if len(u.candidates)<curlen:                 curlen=len(u.candidates)                 curcell = u.dex         c in newboard.board[curcell].candidates:             newboard.board[curcell].candidates = [int(c)]             newboard.board[curcell].value = int(c)             newboard.solve()