Undo stack

Push completed actions to an undo stack.

Undo stack

Now we write a function that pushes an action onto the stack. init_undo comes from the previous lesson and is used here.

The push operation: stack.append(action) adds the action to the end of the stack. Because of LIFO the last added stays on top and is undone first.

We can add several actions and watch the stack grow and its state; this tracks the undo history.

Action list

Actions are represented as strings: 'add', 'delete', 'change'. Each is pushed onto the stack and builds the undo history.

If several actions are added the stack grows. The last added is on top; it is undone first and removed from the history.

The stack size shows how many undos are possible; an empty stack means nothing to undo.

Exercise Push action

Write push_undo(stack, action). Push onto stack, return stack. Print ['ekle', 'sil'] with s = init_undo(); push_undo(s, 'ekle'); push_undo(s, 'sil'); print(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
s = init_undo()
push_undo(s, 'add')
push_undo(s, 'del')
print(s)

Test yourself

Which method does stack push use?
- pop
- insert
- append
- remove
Show answer

append — append

Push return

stack.append(action)
return ____

Show answer

stack.append(action)
return stack

Undo stack: match the terms

append

Push
pop

Extract
LIFO

Last first
Stack

Undo

Next lesson: Pending queue

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