Python: 'str' object has no attribute 'push' in PyCharm

Question

I'm an absolute beginner with Python and generally not very good at coding yet, so hopefully my code isn't too laughable :D

I need to write a Stack Program, where the Stack & the Stack Program is a list. This is my code:

class Stack:
    def __init__(self):
        self.items = []

    def pop(self):
        return self.items.pop()

    def push(self, item):
        self.items.append(item)

    def isEmpty(self):
        return self.items == []

def execute(list):
    result = Stack()
    for x in list:
        if 'LOAD' in x:
            number = substring_after(x, " ")
            result.push(number)
        elif 'ADD' in x:
            result = result.pop() + result.pop()
        elif 'MUL' in x:
            result = result.pop() * result.pop()
        elif 'SUB' in x:
            first = result.pop()
            result = result.pop() - first
        elif 'DIV' in x:
            first = result.pop()
            result = result.pop() / first
        elif 'PRINT' in x:
            print(result.pop())
            if result.isEmpty():
                print('Execution completed')
            else:
                print('There are still values in the Stack')
        else:
            print('Not a valid statement for Stack Program')

def substring_after(s, delim):
    return s.partition(delim)[2]

def main():
    operation1 = ["LOAD 2", "LOAD 3", "ADD", "LOAD 4", "MUL", "PRINT"]
    operation2 = ["LOAD 4", "LOAD 5", "ADD", "LOAD 2", "LOAD 3", "ADD", 
    "MUL", "PRINT"]
    operation3 = ["LOAD 6", "LOAD 7", "LOAD 4", "SUB", "DIV", "LOAD 5", 
    "MUL", "PRINT"]

    execute(operation1)

if __name__ == '__main__':
    main()

The error I'm getting is:

Traceback (most recent call last):
  File "XXX/EA2-Stack.py", line 53, in <module> main()
  File "XXX/EA2-Stack.py", line 47, in main execute(operation1)
  File "XXX/EA2-Stack.py", line 19, in execute result.push(number)
AttributeError: 'str' object has no attribute 'push'

I don't understand:

1.) Why does it say, that there is no 'push'? I defined a method called 'push'. I also tried to write: push(result, number), but then it says "unresolved reference"

2.) Why does it say Attribute push? It's not an attribute, it's a method.

I've got the feeling, that I'm missing something here :( Thanks for helping me :>

Alex Hall · Accepted Answer · 2017-05-24 13:12:16Z

2

result = Stack()
result.push(number)

No problem here.

result = result.pop() + result.pop()
result.push(number)

Now there's a problem! result is now a str (as the error message says) instead of a Stack, and indeed str doesn't have the necessary attribute. Use a separate variable name. And yes, result.push is an attribute, even though it's also a method.

answered May 24, 2017 at 13:12

Alex Hall

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Lorayne Over a year ago

Thanks a lot! I didn't think of this and now it works :)

Łukasz Rogalski · Accepted Answer · 2017-05-24 13:13:08Z

0

Why does it say, that there is no 'push'?

Because you do not call it on Stack instance. After first command executed, result does not point to Stack instance no more.

Why does it say Attribute push? It's not an attribute, it's a method.

Python interpreter does not care yet. First it checks if attribute with this name exists. After that it tries to call it. In Python functions (therefore methods) are first class citizens, there is nothing special about them.

answered May 24, 2017 at 13:13

Łukasz Rogalski

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Comments

Tim S. · Accepted Answer · 2017-05-24 13:19:59Z

0

You are reassigning your result variable to values from the Stack object, thereby destroying your stack object:

result = result.pop() + result.pop()

This destroys your instance of Stack()

answered May 24, 2017 at 13:19

Tim S.

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