local / global lists

[deanhystad]

This is an odd case. You have a global "node_list", and if you left out the assignment at the end of animate_nodes, Python would use the global "node_list" and the code would run (well, eventually). The assignment changes things. The assignment creates a local variable named "node_list".

def animate_nodes():
    for current_node in node_list:  # node_list is a local variable.
        x = current_node[0] + current_node[2]
        y = current_node[1] + current_node[3]
         
        if current_node[2] > 0 and current_node[2] < 10:
            vx = current_node[2] + random.randrange(-1, 1)
             
        if current_node[3] > 0 and current_node[3] < 10:
            vx = current_node[3] + random.randrange(-1, 1)
         
        new_node_list.append = [x, y, vx, vy]
     
    node_list = new_node_list.copy()  # Assignment makes a local node_list

Use "global" to inform python you want to use the global variable

def animate_nodes():
    global node_list
    for current_node in node_list:  # node_list is a local variable.
        x = current_node[0] + current_node[2]
        y = current_node[1] + current_node[3]
         
        if current_node[2] > 0 and current_node[2] < 10:
            vx = current_node[2] + random.randrange(-1, 1)
             
        if current_node[3] > 0 and current_node[3] < 10:
            vx = current_node[3] + random.randrange(-1, 1)
         
        new_node_list.append = [x, y, vx, vy]  # This is the next bug
     
    node_list = new_node_list.copy()  # Assignment makes a local node_list

Better yet, avoid using the global variable. Pass the variable as an argument and return the modified variable. This exposes the purpose of the function instead of hiding it as a side effect.

def animate_nodes(node_list):
    """Where is my docstring?"""
    new_list=[]
    for current_node in node_list:
        x = current_node[0] + current_node[2]
        y = current_node[1] + current_node[3]
        vx = 0  # Default values?
        vy = 0  # Used but never assigned
         
        if current_node[2] > 0 and current_node[2] < 10:
            vx = current_node[2] + random.randrange(-1, 1)
             
        if current_node[3] > 0 and current_node[3] < 10:
            vx = current_node[3] + random.randrange(-1, 1)
         
        new_list.append = [x, y, vx, vy]
     
    return new_list