Python prints return from list pop but when accessing gives 'NoneType' not subscriptable error

Question

I am inserting and popping from a Stack in the following code:

open_stack = util.Stack()

start = []
start.append(problem.getStartState())
open_stack.push(start)

while not open_stack.isEmpty():
    curr_path = open_stack.pop()
    print(type(curr_path))
    if problem.isGoalState(curr_path[-1]):
        return curr_path
    for succ in problem.getSuccessors(curr_path[-1]):
        open_stack.push(curr_path.append(succ[0]))
return False

The print(type(curr_path)) returns:

<class 'list'>
<class 'NoneType'>

The error I get is as follows:

File "/home/ljagodz/uoft/search/search.py", line 101, in depthFirstSearch
if problem.isGoalState(curr_path[-1]):
TypeError: 'NoneType' object is not subscriptable

The Stack class I am using is defined as so:

class Stack:
    "A container with a last-in-first-out (LIFO) queuing policy."

    def __init__(self):
        self.list = []

    def push(self, item):
        "Push 'item' onto the stack"
        self.list.append(item)

    def pop(self):
        "Pop the most recently pushed item from the stack"
        return self.list.pop()

    def isEmpty(self):
        "Returns true if the stack is empty"
        return len(self.list) == 0

I cannot understand why the print is acting this way, and why I am getting this error.. The NoneType error seems common enough, but I cannot find any explanation analogous to the issue I am experiencing here, as far as I can tell I am not accidentally assigning some list method to a variable as in other Stackoverflow questions raising this issue.

Answer 1

The problem most likely comes from

...
for succ in problem.getSuccessors(curr_path[-1]):
    open_stack.push(curr_path.append(succ[0]))

the append method of list will return None object, not the reference to the resulting list. i.e.

print(curr_path.append(succ[0])) 

will print None. So you appended None to your stack. Try:

for succ in problem.getSuccessors(curr_path[-1]):
    open_stack.push(curr_path + [succ[0]]) #use listcomp

also see List Comprehensions

p.s. you can also use list as stack

edit: minor error