Algorithm

Scan each row and find the empty box. Here the Empty boxes are represented by '0'.
Put the first possible Number in that box and move onto the next empty box in the same row.
At one point putting a certain number in a box might not be valid, that moment move back to previous box and delete the number.
Continue this entire process until all of the boxes are filled or in other words there are no '0' in the entire board.

Explaining the Methods

find_space(board): This method helps in finding the nearest empty box or the box containing '0' while scanning every row. If no empty box is found then it returns None else it returns the position of the box.
check(board, num, r, c): This method helps in checking if the number num is already present in the the row r or column c or in the box. This is done in three ways.If the number is found then False is return else True is returned.
1. checking if the number num is present in the row r by scanning through the row.
2. checking if the number num is present in the column c by scanning through the column
3. checking if the number num is present in the box. To do so the first thing to do is doing the integer division of the row r and column c and store it in a couple of variable lets say x and y. The next job is to find the index of the first box of the 3x3 box. This is done by multiplying the variables x and y by 3.
solve(board): This method puts a number in the first found empty box and then checks if the number's position is valid or not. This is done with the help of two methods find_space(board) and check(board,num,r,c).
show(board): This method prints the board in the form of a Sudoku Table/Board.
enter_datas(board): This method takes the values from the puzzle setter.