Boyarsky - Python cs 358

Python sudoku checker

# SPECIFICATION:
#
# check_sudoku() determines whether its argument is a valid Sudoku
# grid. It can handle grids that are completely filled in, and also
# grids that hold some empty cells where the player has not yet
# written numbers.
#
# First, your code must do some sanity checking to make sure that its
# argument:
#
# - is a 9x9 list of lists
#
# - contains, in each of its 81 elements, a number in the range 0..9
#
# If either of these properties does not hold, check_sudoku must
# return None.
#
# If the sanity checks pass, your code should return True if all of
# the following hold, and False otherwise:
#
# - each number in the range 1..9 occurs only once in each row 
#
# - each number in the range 1..9 occurs only once in each column
#
# - each number the range 1..9 occurs only once in each of the nine
#   3x3 sub-grids, or "boxes", that make up the board
#
# This diagram (which depicts a valid Sudoku grid) illustrates how the
# grid is divided into sub-grids:
#
# 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 1
# 7 1 3 | 9 2 4 | 8 5 6
# ---------------------
# 9 6 1 | 5 3 7 | 0 0 0
# 2 8 7 | 4 1 9 | 0 0 0
# 3 4 5 | 2 8 6 | 0 0 0
# 
# Please keep in mind that a valid grid (i.e., one for which your
# function returns True) may contain 0 multiple times in a row,
# column, or sub-grid. Here we are using 0 to represent an element of
# the Sudoku grid that the player has not yet filled in.

# check_sudoku should return None
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],  # <---
              [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]]

# check_sudoku should return True
valid = [[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],
         [4,2,6,8,5,3,7,9,1],
         [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]]

# check_sudoku should return False
invalid = [[5,3,4,6,7,8,9,1,2],
           [6,7,2,1,9,5,3,4,8],
           [1,9,8,3,8,2,5,6,7],
           [8,5,9,7,6,1,4,2,3],
           [4,2,6,8,5,3,7,9,1],
           [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]]

# check_sudoku should return True
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]]

# check_sudoku should return True
hard = [[1,0,0,0,0,7,0,9,0],
        [0,3,0,0,2,0,0,0,8],
        [0,0,9,6,0,0,5,0,0],
        [0,0,5,3,0,0,9,0,0],
        [0,1,0,0,8,0,0,0,2],
        [6,0,0,0,0,4,0,0,0],
        [3,0,0,0,0,0,0,1,0],
        [0,4,0,0,0,0,0,0,7],
        [0,0,7,0,0,0,3,0,0]]

def nine_by_nine(grid):
    if not grid:
        return False
    if len(grid) != 9:
        return False
    for row in grid:
        if len(row) != 9:
            return False
    return True    

def all_digits(grid):
    for row in grid:
        for cell in row:
            try:
                int_value = int(cell)
                if int_value < 0 or int_value > 9 or int_value != cell:
                    return False
            except:
                return False
    return True   


# checks no duplicates from 1 to 9 (duplicate 0's are ok)
def check_no_dups(nineGrouping):  
    uniqueDigits = set()
    for num in nineGrouping:
      if num != 0:
        if num  in uniqueDigits:
	      return False
      uniqueDigits.add(num)
    return True
    

def check_row(grid, row):
    nineGrouping = []
    for cell in grid[row]:
        nineGrouping.append(cell)
    return check_no_dups(nineGrouping)

def check_column(grid, column):
    nineGrouping = []
    for row in grid:
        nineGrouping.append(row[column])
    return check_no_dups(nineGrouping) 

def check_subgrid(grid, row, column):
    nineGrouping = []
    for i in range(row, row + 3):
        for j in range(column, column + 3):
            nineGrouping.append(grid[i][j])
    return check_no_dups(nineGrouping)

def check_sudoku(grid):
    if not nine_by_nine(grid) or not all_digits(grid):
      return None
    for i in range(0, 9):
      if not check_row(grid, i):
          return False
      if not check_column(grid, i):
	      return False
    for i in range(0, 7, 3):
	  for j in range(0, 7, 3):
	    if not check_subgrid(grid, i, j):
		  return False

    return True
    pass

print check_sudoku(ill_formed) # --> None
print check_sudoku(valid)      # --> True
print check_sudoku(invalid)    # --> False
print check_sudoku(easy)       # --> True
print check_sudoku(hard)       # --> True

Please clarify for me. What's the use of int_value != cell: on line 118? A string and an int can't be the same. If the cast weren't possible an error would have been produced on previous line 117 with int_value = int(cell).