what is " if not self.list_stack: " mean? why it can check if the stack is empty or not?

Hey, Thank you for your question! Let me answer it by walking you through the following example. In a new cell in the notebook define an empty list as follows: list_stack = [] Printing out the list simply returns an empty array. Now, however, try to convert the output in a Boolean format as follows: bool(list_stack) Executing this line should output False. Therefore, an empty list converted into Boolean outputs False. Now, add an element inside the list, for example as follows: list_stack = [2] Convert again the list into a Boolean and, this time, the output should be True. Therefore, a non-empty list returns True. Let's now consider Giles' code. The first line of the is_empty() method reads if not self.list_stack: Since the if-statement can only check Boolean values (whether a condition is True or False), self.list_stack will be a Boolean expression. For an empty list, self.list_stack would return False. The not command in front would turn False into True. Therefore, for an empty list, the expression if not self.list_stack: would turn into if not False: which, in turn, becomes if True: Therefore, we enter the if-statement and return True. This is great, as the is_empty() method returns True and we indeed started with an empty list. Consider now a non-empty list. Then if not self.list_stack: becomes if not True: which is equivalent to if False: Therefore, we don't enter the if-statement and jump straight to the else-statement. The is_empty() method returns False. This, again, is indeed what we want, since we started with a non-empty list. Hope the answer helps! Kind regards, 365 Hristina

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I-Cheng Liu

Last answered:

11 Apr 2022

Posted on:

10 Apr 2022

what is " if not self.list_stack: " mean?

in Python Programmer Bootcamp / Coding a stack in Python

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Hristina Hristova

Instructor

Posted on:

11 Apr 2022

what is " if not self.list_stack: " mean?

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