These two achieve the exact same thing:
```
if not days:
    days = self.days
```
as
```
days = days or self.days
```

For example:

```
>>> days= ''
>>> default = "DEFAULT"
>>> days or default
'DEFAULT'
>>> days= 'Something'
>>> days or default
'Something'
```

In conclusion:

* The single line statement is shorter :)
* By just asking `if not days` or using my more "clever" boolean expression, it checks if `days` is *anything* useless such as `None`, `''` or 'False`.