AttributeError: 'list' object has no attribute '_Stack__items'

Question

I have a class Stack in which I implemented a function which reverses a string.

class Stack:

        def __init__(self,x):
            self.__items = []


        def reverse(self):
            if len(self.__items ) <= 1:
                return self.__items
            else:
                return reverse(self.__items [1:]) + self.__items [0]        



word=input("Please enter word to reverse: ")
list_word=list(word)
rev=Stack.reverse(list_word)
print(rev)

I have a fully functioning reverse function but yet my class returns me an Attribute Error. Thoughts? Do I need a for loop?

Scott Harris · Accepted Answer · 2015-01-30 03:29:21Z

If you want to call this as a static method, the following should work:

@staticmethod
def reverse(the_list):
    if len(the_list) <= 1:
        return the_list
    else:
        return reverse(the_list[1:]) + [the_list[0]]

Ultimately though, I think you're misunderstanding the differences between static methods and instance methods.

Instance Methods

Instance methods can only be called on an instance of an object. In the case of your Stack class, you would have to call the method like so:

stack = Stack()
stack.reverse([1, 2, 3, 4])

First, we have to create an instance of the Stack class, and then we call its instance method reverse. With an instance method, the method signature for reverse will look like the following:

def reverse(self, the_list):

Note that there are two arguments listed in the method signature, even though we only passed it one argument when we called it above. This is because, for an instance method, there is an implicit self argument that represents the instance of the object that we're calling the method on. Any additional parameters will be bound to the subsequent arguments. Our list, [1, 2, 3, 4], is bound to the the_list parameter of the function.

Contrast this with...

Static Methods

A static method is a method attached to the class itself, rather than an instance of that class. Thus, you don't need an instance in order to call the method. That's what you've got in your question:

#calling the method statically
Stack.reverse([1, 2, 3, 4])

Within the Stack class, the method signature for reverse will look like the following:

@staticmethod
def reverse(the_list):

Note the difference: there's no self parameter anymore. This is because a static method is just like a normal function, but it's attached to a class. Because it's a normal function, there's no object instance associated with it and, therefore, no self parameter.

Here's a better explanation: https://julien.danjou.info/blog/2013/guide-python-static-class-abstract-methods

Avi Mosseri · Accepted Answer · 2015-01-30 03:11:27Z

0

stack.reverse is not a static method. you have to define a stack object and run the function on the object. For future reference, you can use python's @staticmethod keyword on top of the method if it doesn't depend on any class variables to treat it like any other function.

edited Jan 30, 2015 at 3:11

answered Jan 30, 2015 at 3:06

Avi Mosseri

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2 Comments

user2555451 Over a year ago

Making reverse a staticmethod will not work in this case. The method needs access to self.__items. Remember that staticmethods do not receive a self argument.

Avi Mosseri Over a year ago

true, should have taken a closer look

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