Reading week

Review? Maybe

return str.join(s, [nested_join(s, x) if isinstance(x, list) else x for x in L])

Empty Stack: (last in, first out) | | | | | | ------- After Pushing A, you get: | | | | | A | ------- After Pushing B, you get: | | | A | | B | ------- After Popping, you get: | | | | | B | ------- class Stack:     """A last-in, first-out (LIFO) stack of items"""     def __init__(self: 'Stack', data: list =None) -> None:         """Create a new empty Stack using data         data --- initial contents of Stack         """         if data:              self._data = data[:]          else:             self._data = []     def pop(self: 'Stack') -> object:         """Remove and return the top item."""         return self._data.pop()     def is_empty(self: 'Stack') -> bool:         """Return whether the stack is empty."""         return self._data == []     def push(self: 'Stack', obj: object) -> None:         """Place o on top of the stack."""         self._data.append(obj)