A star path finding explanation in python

gistfile1.txt

# The total cost of a node is calculated as 

# f(n) = g(n) + h(n)

# g(n) is the cost from the start node to the current node, and
# h(n) is the estimated cost from the current node to the goal.
# f value is the sum of g and h and is used to compare nodes when deciding

#  the parent attribute of a Node 
#  is used to keep track of the path 
# from the start node to the current node.

class Node:
    def __init__(self, position=None, parent=None):
        self.position = position
        self.parent = parent
        self.g = 0
        self.h = 0
        self.f = 0

    # Compare nodes
    def __eq__(self, other):
        return self.position == other.position
    
######################################################

# astar is the main function
# start is where the node begins and end_node is the goal

#open list is for nodes that have been discovered but not evaluated. It begins
#with the start node, new nodes (neighbors of the currently evaluated nodes) are added to this list.
#these are potential candidates for path continuation
#the algorithm will judge based on their f values.

#closed list is for nodes whose neighbours f values have been evaluated
#The algorithm terminates when the goal node is added to the closed list

def astar(grid, start, end):

    # Create start and end node
    start_node = Node(start, None)
    end_node = Node(end, None)
    print(f"Start: {start_node.position}")
    print(f"End: {end_node.position}") 
     

    # Initialize open and closed list
    open_list = []
    closed_list = []
    
    # Add the start node
    open_list.append(start_node)

    # Loop until the end node is found
    while len(open_list) > 0:
      
        # print('open', [node.position for node in open_list])

        # Get the current node (node with the lowest f value)
        current_node = open_list[0]
        current_index = 0
        for index, item in enumerate(open_list):
            if item.f < current_node.f:
                current_node = item
                current_index = index

        # Pop current node from open list and add to closed list
        open_list.pop(current_index)
        closed_list.append(current_node)

        # Check if we reached the end
        if current_node == end_node:
            path = []
            current = current_node
            while current is not None:
                path.append(current.position)
                current = current.parent
            return path[::-1]  # Return reversed path

        # Generate children
        children = []
        for new_position in [(0, -1), (0, 1), (-1, 0), (1, 0)]:  # Adjacent squares
            node_position = (current_node.position[0] + new_position[0], current_node.position[1] + new_position[1])

            # Make sure within range
            if node_position[0] > (len(grid) - 1) or node_position[0] < 0 or node_position[1] > (len(grid[len(grid)-1]) -1) or node_position[1] < 0:
                continue

            # Make sure walkable
            if grid[node_position[0]][node_position[1]] != 0:
                continue

            # Create new node and set current node as parent
            new_node = Node(node_position, current_node)

            # Append
            children.append(new_node)

        # Loop through children
        for child in children:
            # Child is on the closed list
            if len([closed_child for closed_child in closed_list if closed_child == child]) > 0:
                continue

            # Create the f, g, and h values
            child.g = current_node.g + 1
            child.h = ((child.position[0] - end_node.position[0]) ** 2) + ((child.position[1] - end_node.position[1]) ** 2)
            child.f = child.g + child.h

            # Child is already in the open list
            if len([open_node for open_node in open_list if child == open_node and child.g > open_node.g]) > 0:
                continue

            # Add the child to the open list
            open_list.append(child)

    return None  # No path found

# Example usage
grid = [[0, 0, 0, 0, 0],
        [0, 1, 1, 1, 0],
        [0, 1, 0, 0, 0],
        [0, 0, 0, 1, 1],
        [0, 0, 0, 0, 0]]

start = (0, 0)
end = (4, 4)

path = astar(grid, start, end)
#print(path)