Pending queue

Keep pending tasks in a queue.

Pending queue

Now we write the undo operation. pop_undo removes and returns the last action from the stack; this performs the undo.

If the stack is empty there is nothing to undo. In that case a user-friendly message is returned and an error is avoided.

The pop operation: stack.pop(). It removes and returns the last element. Thanks to LIFO the most recent action is undone.

Empty stack check

Before popping we must check whether the stack is empty. Pop on an empty stack raises an error and may crash the program.

In Python if not stack: checks for an empty list. If empty return a message, otherwise pop. This provides a safe undo.

This check is essential for a user-friendly and error-free undo system; it must always be done.

Exercise Undo

Write pop_undo(stack). 'boş' if empty, else pop. Print ekle with s = init_undo(); push_undo(s, 'ekle'); print(pop_undo(s)).

Show example solution

def init_undo():
    # Empty undo stack
    return []
def push_undo(stack, action):
    # Push to undo stack
    stack.append(action)
    return stack
def pop_undo(stack):
    # Pop from undo stack
    if not stack:
        return 'boş'
    return stack.pop()
s = init_undo()
push_undo(s, 'add')
print(pop_undo(s))

Test yourself

Pending queue: match the terms

Pop

Remove
Empty stack

Message
LIFO

Last first
Undo

Reverse
What should pop do on an empty stack?
- Raise error
- Take the last
- Crash
- Return a message
Show answer

Return a message — Return a message
Pop operation
```
return stack.____()
```
Show answer
```
return stack.pop()
```

Next lesson: Print the state

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