I don't understand the Deck class in Milestone Project 2

In particular what is bold here:

```
def __str__(self):
        deck_comp = ""
        for card in **self.cards**:
            **deck_comp += " " + deck_comp.__str__()**

        return "The deck has" + deck_comp
```
self.cards - how is self.cards known?
deck_comp.__str__() seems like a method for the general string function?

Thanks in advance.

A frustrated user.