A simple backtracking algorithm for solving a Maze in Python

Backtrack.py

OBJECTS = {'WALL': '#',
           'SPACE': ' ',
           'VISITED': '.', }


def readfile(file):
    try:
        f = open(file, 'r')
    except Exception as e:
        print('Error opening file. Did you enter the correct file-path?')
        print(e)

    # read each line
    grid = [line.replace('\n', '').split(',') for line in f]
    return grid


def check_matrix(grid):
    for i in range(0, len(grid)):
        for j in range(0, len(grid[i])):
            # if there is an undefined character, something is most likely going to be wrong
            if not grid[i][j] in OBJECTS.values():
                return False
    return True


def valid(x, y, grid):
    # to be valid the position has to be inside the grid and not yet been visited
    return 0 <= x < len(grid[0]) and 0 <= y < len(grid) and grid[y][x] is OBJECTS['SPACE']


def find_path(grid, x, y, endx, endy, path):
    # destination reached ?
    if x == endx and y == endy:
        return path
    # current position must be valid
    if not valid(x, y, grid):
        return None
    # mark current position
    grid[y][x] = OBJECTS['VISITED']

    # try every possible direction
    # north
    if find_path(grid, x, y - 1, endx, endy, path) is not None:
        path.append('N')
        return path
    # south
    if find_path(grid, x, y + 1, endx, endy, path) is not None:
        path.append('S')
        return path
    # east
    if find_path(grid, x + 1, y, endx, endy, path) is not None:
        path.append('E')
        return path
    # west
    if find_path(grid, x - 1, y + 1, endx, endy, path) is not None:
        path.append('W')
        return path
    # every possible step was invalid
    # so just take a step back
    if len(path) > 0:
        path.remove(len(path)-1)
    grid[y][x] = OBJECTS['SPACE']


grid = readfile('test1.txt')
check_matrix(grid)
path = find_path(grid, 1, 1, 3, 3, [])
if path is None:
    path = []
print(path[len(path)-1:0:-1])

test1.txt

#,#,#,#,#,#,#,#,#,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , ,#, , , ,#
#, , , , , , , , ,#
#,#,#,#,#,#,#,#,#,#

test2.txt

 ,#,#,#,
 , ,#, ,
#, ,#, ,
 , , , ,