I have a class:

class AlchemicalElement:

    def __init__(self, name: str):
        self.name = name

I then create a class that will be used to store the AlchemicalElement objects:

class AlchemicalStorage:

    def __init__(self):
        self.storage_list = []

I don't understand how to write this function:

def extract(self) -> list[AlchemicalElement]:
    """Return a list of all the elements from storage and empty the storage itself."""
    return []

So far I've only managed this:

def add(self, element: AlchemicalElement):
    if isinstance(element, AlchemicalElement):
        self.storage_list.append(element)
    else:
        raise TypeError()

This is what I try to achieve:

    storage = AlchemicalStorage()
    storage.add(AlchemicalElement('Water'))
    storage.add(AlchemicalElement('Fire'))
    storage.extract() # -> [<AE: Water>, <AE: Fire>]
    storage.extract() # -> []

Question

How do I write the .extract() method so that when first called it returns a list of elements in the storage_list, but an empty list on any call after?

CodePudding user response：

Assuming this is what you're starting with

class AlchemicalElement:

    def __init__(self, name: str):
        self.name = name


class AlchemicalStorage:

    def __init__(self):
        self.storage_list = []

    def add(self, element: AlchemicalElement):
        if isinstance(element, AlchemicalElement):
            self.storage_list.append(element)
        else:
            raise TypeError()

    def extract(self) -> list[AlchemicalElement]:
        """Return a list of all the elements from storage and empty the
        storage itself."""
        return []


if __name__ == "__main__":
    storage = AlchemicalStorage()
    storage.add(AlchemicalElement('Water'))
    storage.add(AlchemicalElement('Fire'))
    print(storage.extract())  # -> [<AE: Water>, <AE: Fire>]
    print(storage.extract())  # -> []

You'll currently get output that looks like this

[]
[]

Your first question is how do I make the .extract() method exhaust the .storage_list?

We could do something like this:

    def extract(self) -> list[AlchemicalElement]:
        """Return a list of all the elements from storage and empty the
        storage itself."""
        # copy the storage_list into out_list and then clear it
        # only works once!
        out_list = self.storage_list.copy()
        self.storage_list.clear()
        return out_list

Now if we run the script we should get the following:

[<__main__.AlchemicalElement object at 0x000001CB12CBBD00>, <__main__.AlchemicalElement object at 0x000001CB12CBBCA0>]
[]

Now maybe to answer another question, how to do I get it to return <AE: Water> instead of <__main__.AlchemicalElement object at ...>?

We can do that by adding a __repr__ method to the AlchemicalElement class

class AlchemicalElement:

    def __init__(self, name: str):
        self.name = name

    def __repr__(self):
        return f"<AE: {self.name}>"

All code together now looks like this

class AlchemicalElement:

    def __init__(self, name: str):
        self.name = name

    def __repr__(self):
        return f"<AE: {self.name}>"


class AlchemicalStorage:

    def __init__(self):
        self.storage_list = []

    def add(self, element: AlchemicalElement):
        if isinstance(element, AlchemicalElement):
            self.storage_list.append(element)
        else:
            raise TypeError()

    def extract(self) -> list[AlchemicalElement]:
        """Return a list of all the elements from storage and empty the
        storage itself."""
        # copy the storage_list into out_list and then clear it
        # only works once!
        out_list = self.storage_list.copy()
        self.storage_list.clear()
        return out_list


if __name__ == "__main__":
    storage = AlchemicalStorage()
    storage.add(AlchemicalElement('Water'))
    storage.add(AlchemicalElement('Fire'))
    print(storage.extract())  # -> [<AE: Water>, <AE: Fire>]
    print(storage.extract())  # -> []

And output looks like how you want

[<AE: Water>, <AE: Fire>]
[]

Note:

The __repr__ method should return a printable representation of the object, most likely one of the ways possible to create this object. source