Invalid syntax error - Python

[BerhtSammet]

Hola
Pues bien tengo un problema con mi programa, al momento de compilarlo me marca error en una linea del main, especificamente en estas y no entiendo porque

Código Python:

print ("______________________________________________________________________")
    print ('BFS Path')
    print (BFS(1,3,GRAPH))
 

Soy nueva en esto de la programación con Python y aunque he buscado información no logro comprendeer el error.

Les dejo el código:

Código Python:

#import pygraphviz as pgv
 
 
def BFS(start, target, GRAPH):
    'Use a QUEUE to search.'
    print ("Source:",source)
    print("Target:",target)
    queue = [start]
    visited = []
 
    while len(queue) > 0:
      x = queue.pop(0)
 
      if x == target:
        visited.append(x)
        return visited
      elif x not in visited:
        visited = visited+[x]
        if GRAPH[x] is not None:
           'add nodes at the END of the queue'
           queue = queue + GRAPH[x]
 
    return visited
 
def DFS(start, target, GRAPH):
    'Use a STACK to search.'
    print ("Source:",source)
    print("Target:",target)
    stack = [start]
    visited = []
 
    while len(stack) > 0:
      x = stack.pop(0)
 
      if x == target:
        visited.append(x)
        return visited
      elif x not in visited:
        visited = visited+[x]
        if GRAPH[x] is not None:
           'add nodes at the top of the stack'
           stack = GRAPH[x] + stack
 
    return visited
 
 
def main():
 
    GRAPH = {1 : [2,3], 2:[4,5], 3:[6], 4:None, 5:[7,8], 6:None, 7:None, 8:None}
 
 
    print ("BFS Path",BFS(1,7,GRAPH))
    print ("DFS Path",DFS(1,7,GRAPH)
    print ("______________________________________________________________________")
    print ('BFS Path')
    print (BFS(1,3,GRAPH))
    print ("DFS Path",DFS(1,3,GRAPH))
 

Gracias!