Python stacks and queues

A stack is last in, first out: a list's append() and pop(). A queue is first in, first out: collections.deque with append() and popleft().

Stacks match brackets and undo actions; queues serve things in the order they came.

stack = []
for ch in "(()":
    if ch == "(":
        stack.append(ch)
    elif stack:
        stack.pop()
print("left open:", len(stack))

Watch for: pop() on an empty list stops the program: check the stack isn't empty first.