In this lab a grocery list editor with undo functionality is implemented.

Step $1$: Inspect the UndoCommand abstract base class

The read$-$only

UndoCommand.py file has a declaration for the UndoCommand abstract base class. Access

UndoCommand.py by clicking

on the orange arrow next to

main.py at the top of the coding window. The UndoCommand class represents a command object: an object

that stores all needed information to execute an action at a later point in time. For this lab, a command object stores information to undo a

grocery list change made by the user.

Step $2$: Inspect the incomplete GroceryList class

The GroceryList class is declared in

GroceryList.py$.$ Two attributes are declared:

A list of strings for list items

A stack of UndoCommand references for undo commands

A simple list implementation of stack ADT is provided in

Stack.py$.$

Note that the add$\_$with$\_$undo$()$ method is already implemented. The method adds a new item to the list and pushes a new

RemoveLastCommand object onto the undo stack.

Step $3$: Implement RemoveLastCommand's execute$()$ method

The RemoveLastCommand class inherits from UndoCommand and is declared in

RemoveLastCommand.py$.$ When a

RemoveLastCommand object is executed, the string list's last element is removed. So when the user appends a new item to the grocery

list, a RemoveLastCommand is pushed onto the stack of undo commands. Popping and executing the RemoveLastCommand then

removes the item most recently added by the user.

RemoveLastCommand's source$\_$list attribute and constructor are already declared:

source$\_$list is a reference to a GroceryList object's list of strings.

The constructor takes a reference to a list of strings as a parameter, and assigns source$\_$list with the reference.

Implement RemoveLastCommand's execute$()$ method to remove source$\_$list's last element.