Sudoku Solver in Python
About the project: This is a project for a Sudoku Solver in Python. This script will solve any valid Sudoku puzzle using a classic backtracking algorithm, which is a powerful technique for solving this type of problem.
The project is a single, self-contained Python file. It includes a sample puzzle and functions to solve it and display the results neatly on the console.
The solve_sudoku function uses a backtracking algorithm. This is a powerful technique that tries to build a solution incrementally. If a path leads to a dead end (a number can't be placed), the algorithm "backtracks" to the previous choice and tries a different option until it finds a valid path to the solution.
Project Level: Intermediate
You can directly copy the below snippet code with the help of green copy button, paste it and run it in any Python editor you have.
Steps: Follow these stepsStep 1: Copy below code using green 'copy' button.
Step 2: Paste the code on your chosen editor.
Step 3: Save the code with filename and .py extention.
Step 4: Run (Press F5 if using python IDLE)
# sudoku_solver.py
def print_board(board):
"""
Prints the Sudoku board in a clean, readable format.
Adds lines to separate the 3x3 sub-grids.
"""
for i, row in enumerate(board):
if i % 3 == 0 and i != 0:
print("- - - - - - - - - - - -")
for j, val in enumerate(row):
if j % 3 == 0 and j != 0:
print(" | ", end="")
if j == 8:
print(val)
else:
print(str(val) + " ", end="")
def find_empty_location(board):
"""
Finds the next empty location (a cell with value 0) on the board.
Args:
board (list of lists): The 9x9 Sudoku board.
Returns:
tuple or None: A tuple (row, col) of the empty location, or None if no empty cells exist.
"""
for row in range(9):
for col in range(9):
if board[row][col] == 0:
return (row, col)
return None
def is_valid_move(board, row, col, num):
"""
Checks if it's safe to place a number at a given location.
Args:
board (list of lists): The 9x9 Sudoku board.
row (int): The row index.
col (int): The column index.
num (int): The number to check (1-9).
Returns:
bool: True if the move is valid, False otherwise.
"""
# Check if the number is already in the row
if num in board[row]:
return False
# Check if the number is already in the column
for r in range(9):
if board[r][col] == num:
return False
# Check if the number is in the 3x3 sub-grid
start_row = row - row % 3
start_col = col - col % 3
for r in range(start_row, start_row + 3):
for c in range(start_col, start_col + 3):
if board[r][c] == num:
return False
return True
def solve_sudoku(board):
"""
The main Sudoku solver function using a backtracking algorithm.
Args:
board (list of lists): The 9x9 Sudoku board.
Returns:
bool: True if a solution is found, False if the puzzle is unsolvable.
"""
# Find the next empty cell
find = find_empty_location(board)
# Base case: If no empty cells, the puzzle is solved.
if not find:
return True
else:
row, col = find
# Recursive case: Try placing numbers 1-9 in the empty cell.
for num in range(1, 10):
if is_valid_move(board, row, col, num):
# If the number is valid, place it
board[row][col] = num
# Recursively call the solver. If it finds a solution, we're done.
if solve_sudoku(board):
return True
# If the recursive call fails, backtrack and reset the cell to 0
board[row][col] = 0
# If no number from 1-9 works, this path is a dead end. Return False to backtrack.
return False
# Example usage
if __name__ == "__main__":
# A sample Sudoku puzzle (0 represents an empty cell)
# This board is from a real Sudoku puzzle.
example_board = [
[5, 3, 0, 0, 7, 0, 0, 0, 0],
[6, 0, 0, 1, 9, 5, 0, 0, 0],
[0, 9, 8, 0, 0, 0, 0, 6, 0],
[8, 0, 0, 0, 6, 0, 0, 0, 3],
[4, 0, 0, 8, 0, 3, 0, 0, 1],
[7, 0, 0, 0, 2, 0, 0, 0, 6],
[0, 6, 0, 0, 0, 0, 2, 8, 0],
[0, 0, 0, 4, 1, 9, 0, 0, 5],
[0, 0, 0, 0, 8, 0, 0, 7, 9]
]
print("--- Unsolved Sudoku Board ---")
print_board(example_board)
print("\nAttempting to solve...\n")
if solve_sudoku(example_board):
print("--- Solved Sudoku Board ---")
print_board(example_board)
else:
print("No solution exists for this Sudoku puzzle.")
← Back to Projects
 in Python){kind=link}