Return to Answer

Note the use of _ for throw-away variables.

Note the use of _ for throw-away variables.

from itertools import chain
def is_valid_block(block):
 """Returns true if a block (row, column or square) is a valid Sudoku block."""
 return len(block) == 9 and sum(block) == sum(set(block))

def is_valid_sudoku(matrix):
 """Returns True if a Sudoku matrix is valid, False otherwise."""
 # check each row
 for row in matrix:
 if not is_valid_block(row):
 return False
 # check each column
 for col in zip(*matrix):
 if not is_valid_block(col):
 return False
 # check each 3x3 square
 for i in range(0, 9, 3):
 for j in range(0, 9, 3):
 square = list(chain(row[j:j + 3] for row in matrix[i:i + 3]))
 if not is_valid_block(square):
 return False
 return True

• use pretty-printing via pprint instead of a regular print
• unused square_x and square_y variables in the "main" part of the script
• organize imports per PEP8 (reference)
• two blank lines between the functions (reference)

def is_valid_block(block):
 """Returns true if a block (row, column or square) is a valid Sudoku block."""
 return len(block) == 9 and sum(block) == sum(set(block))

from itertools import chain
def is_valid_sudoku(matrix):
 """Returns True if a Sudoku matrix is valid, False otherwise."""
 # check each row
 for row in matrix:
 if not is_valid_block(row):
 return False
 # check each column
 for col in zip(*matrix):
 if not is_valid_block(col):
 return False
 # check each 3x3 square
 for i in range(0, 9, 3):
 for j in range(0, 9, 3):
 square = list(chain(row[j:j + 3] for row in matrix[i:i + 3]))
 if not is_valid_block(square):
 return False
 return True

• use pretty-printing via pprint instead of a regular print
• unused square_x and square_y variables in the "main" part of the script (that might be actually an error/bug, since, I was expecting you to use them to make a key for the square_map dictionary)
• organize imports per PEP8 (reference)
• two blank lines between the functions (reference)

We can also take it a step further and make use of generators and any():

def generate_blocks(matrix):
 """Generates rows, columns and 3x3 squares for a Sudoku matrix."""
 for row in matrix:
 yield row
 for col in zip(*matrix):
 yield col
 
 for i in range(0, 9, 3):
 for j in range(0, 9, 3):
 yield list(chain(*(row[j:j + 3] for row in matrix[i:i + 3])))
def is_valid_sudoku(matrix):
 """Returns True if a Sudoku matrix is valid, False otherwise."""
 return any(not is_valid_block(block) for block in generate_blocks(matrix))

I have never written a sudoku solver before, but here is how I would think about implementing it:

We can also take it a step further and make use of generators and all():

def generate_blocks(matrix):
 """Generates rows, columns and 3x3 squares for a Sudoku matrix."""
 for row in matrix:
 yield row
 for col in zip(*matrix):
 yield col
 
 for i in range(0, 9, 3):
 for j in range(0, 9, 3):
 yield list(chain(*(row[j:j + 3] for row in matrix[i:i + 3])))
def is_valid_sudoku(matrix):
 """Returns True if a Sudoku matrix is valid, False otherwise."""
 return all(is_valid_block(block) for block in generate_blocks(matrix))

I have never written a sudoku solver before, but here is how I would think about implementing the brute-force solution (I guess that is just a different approach and it's not related to the code review):