[array_stack.py code is given below]

class ArrayStack: """LIFO Stack implementation using a Python list as underlying storage.""" def __init__(self): """Create an empty stack.""" self._data = [] # nonpublic list instance def __len__(self): """Return the number of elements in the stack.""" return len(self._data) def is_empty(self): """Return True if the stack is empty.""" return len(self._data) == 0 def push(self, e): """Add element e to the top of the stack.""" self._data.append(e) # new item stored at end of list def top(self): """Return (but do not remove) the element at the top of the stack. Raise Empty exception if the stack is empty. """ if self.is_empty(): raise Empty('Stack is empty') return self._data[-1] # the last item in the list def pop(self): """Remove and return the element from the top of the stack (i.e., LIFO). Raise Empty exception if the stack is empty. """ if self.is_empty(): raise Empty('Stack is empty') return self._data.pop() # remove last item from list if __name__ == '__main__': S = ArrayStack() # contents: [ ] S.push(5) # contents: [5] S.push(3) # contents: [5, 3] print(len(S)) # contents: [5, 3]; outputs 2 print(S.pop()) # contents: [5]; outputs 3 print(S.is_empty()) # contents: [5]; outputs False print(S.pop()) # contents: [ ]; outputs 5 print(S.is_empty()) # contents: [ ]; outputs True

Implement a program that can input an expression in postfix notation (see following explanation) and output its value, using stack from the class ArrayStack in file array_stack.py. Postfix notation is an unambiguous way of writing an arithmetic expression without parentheses. It is defined so that if (expl)op(exp2) is a normal, fully parenthesized expression whose operation is op, the postfix version of this is pexp pexp2 op, where pexp, is the postfix version of exp, and pexp2 is the postfix version of exp2. The postfix version of a single number or variable is just that number or variable. For example, the postfix version of "((5+2)*(8-3))/4" is "52 +8 3 - *4/