title: Sudoku Checker in Python
body: <p>I am trying to create a sudoku checker in python:</p>
  
  <pre><code>ill_formed = [[5,3,4,6,7,8,9,1,2],
                [6,7,2,1,9,5,3,4,8],
                [1,9,8,3,4,2,5,6,7],
                [8,5,9,7,6,1,4,2,3],
                [4,2,6,8,5,3,7,9],  # &lt;---
                [7,1,3,9,2,4,8,5,6],
                [9,6,1,5,3,7,2,8,4],
                [2,8,7,4,1,9,6,3,5],
                [3,4,5,2,8,6,1,7,9]]
  easy = [[2,9,0,0,0,0,0,7,0],
         [3,0,6,0,0,8,4,0,0],
         [8,0,0,0,4,0,0,0,2],
         [0,2,0,0,3,1,0,0,7],
         [0,0,0,0,8,0,0,0,0],
         [1,0,0,9,5,0,0,6,0],
         [7,0,0,0,9,0,0,0,1],
         [0,0,1,2,0,0,3,0,6],
         [0,3,0,0,0,0,0,5,9]]
  </code></pre>
  
  <p>I am expecting input like that- a list of 9 lists. The zeros represent number that have not been filled in by the user. They can appear multiple times in a row, column or 3x3.</p>
  
  <pre><code>def check_sudoku(grid):
  if len(grid) == 9:
      numsinrow = 0
      for i in range(9):
          if len(grid[i]) == 9:
              numsinrow += 1
      if numsinrow == 9:
          for i in range(9):
              rowoccurence = [0,0,0,0,0,0,0,0,0,0]
              for j in range(9):
                  rowoccurence[grid[i][j]] += 1
                  temprow = rowoccurence[1:10]
                  if temprow == [1,1,1,1,1,1,1,1,1]:
                      return True
                  else:
                      return False
      else:
          return False
  else:
      return False
  </code></pre>
  
  <p>I obviously need to check that there is a 9x9 list of lists (grid), and that there are no duplicates in each row, column and 3x3 small square. In the code, I first check to see if there are a proper number of rows (There should be 9). Then I check that each row has 9 elements in it (with the ill_formed example you see that this is not the case). I then attempt to check duplicates in each row but I am having some trouble doing so. I thought that I could loop over each row and loop over each element in that row, and add 1 to a list of ints (rowoccurence). For example, if the first number is a 2, then rowoccurence[2] should be equal to 1. The zeros are in rowoccurence[0] and are not checked(I have a temporary list which should take everything except that first element- the zeros- because there could be more than 1 zero in a row and the grid could still be legit). I try to check the temp list (basically rowoccurence) against a reference list of correct values but it does not seem to be working. Could you help me check the rows for duplicates in this sudoku checker? Thank you so much in advance!</p>