I have two lists of strings. When I cast two lists into `set()` then subtraction works properly.

```
>>> A = ['One', 'Two', 'Three', 'Four']
>>> B = ['One']
>>> print(list(set(A) - set(B)))
['Three', 'Two', 'Four']

```

However when variable `B` is a string and I cast it into `set()` then, the subtraction is not working.

```
>>> A = ['One', 'Two', 'Three', 'Four']
>>> B = 'One'
>>> print(list(set(A) - set(B)))
['One', 'Two', 'Three', 'Four']

```

Is anyone able to explain me if its a bug or expected behavior?