Skeleton code for Sudoku solver in Python

sudoku.py

from typing import (
    List,
    TypeVar,
)

# Type aliases
A = TypeVar('A')
Digit = int
Matrix = List[List[A]]
Grid = Matrix[Digit]


# Some grids
grid1 = [[0, 2, 4, 0],
         [1, 0, 0, 3],
         [4, 0, 0, 2],
         [0, 1, 3, 0]]  # type: Grid

grid2 = [[0, 3, 0, 1],
         [1, 0, 3, 2],
         [3, 0, 1, 0],
         [0, 1, 0, 3]]  # type: Grid


# Define grid size
box_size = 2
grid_size = box_size * box_size


# Exercise 1
def solve(grid: Grid) -> List[Grid]:
    pass


# Exercise 2
def completions(grid: Grid) -> List[Grid]:
    pass


# Exercise 3
def choices(grid: Grid) -> Matrix[List[Digit]]:
    pass


# Exercise 4
def expand(matrix: Matrix[List[Digit]]) -> List[Grid]:
    pass


# Exercise 5
def cp(xss: List[List[A]]) -> List[List[A]]:
    pass


# Exercise 6
def valid(grid: Grid) -> bool:
    pass


# Exercise 7
def nodups(xs: List[A]) -> bool:
    pass


# Exercise 8
def rows(grid: Matrix[A]) -> Matrix[A]:
    pass


# Exercise 9
def cols(grid: Matrix[A]) -> Matrix[A]:
    pass


# Exercise 10
def boxs(grid: Matrix[A]) -> Matrix[A]:
    pass


# Exercise 11
def group(xs: List[A]) -> List[List[A]]:
    pass


# Exercise 12
def ungroup(xss: List[List[A]]) -> List[A]:
    pass