Python code to test if the input is a valid Sudoku puzzle

check_sudoku.py

from collections import Counter

def sanity_check(grid):
    if not(isinstance(grid,list)
           and len(grid) == 9
           and all(isinstance(row, list)
                   and len(row) == 9
                   for row in grid)
           and all(isinstance(elt, int)
                   and 0 <= elt <= 9
                   for row in grid
                   for elt in row)):
        return None
    return True

def all_different(line):
    freq = Counter(line)
    return all(freq[x] == 1
               for x in freq
               if x > 0)

def get_row(grid, index):
    return grid[index]

def get_column(grid,index):
    return [row[index]
            for row in grid]

def get_block(grid,index):
    r = 3 * (index / 3)
    c = 3 * (index % 3)
    return grid[r][c:c+3] + grid[r+1][c:c+3] + grid[r+2][c:c+3]
    
def check_sudoku(grid):
    if sanity_check(grid) is None:
        return None
    return all(
        all_different(get_row(grid, k)) and
        all_different(get_column(grid, k)) and
        all_different(get_block(grid, k))
        for k in range(9))