I have a dataframe as below:

```
df = pd.DataFrame({'a':[1,2,3,4], 
                   'b':["west, east", "east, north","south, west","east, south"]})

   a            b                                                                                                                     
0  1   west, east                                                                                                                     
1  2  east, north                                                                                                                     
2  3  south, west                                                                                                                     
3  4  east, south            

```

I want to get unique strings from column `b` like below.

**Expected Output:**

```
["east", "west", "north", "south"]   # order doesn't matter here

```

**My Efforts**

```
op = []
for _, value in df['b'].items():
    op.extend(value)

op = set(op)

```

Which is giving me correct result but is there more efficient way to do so?

My original dataset has around million rows and 1000s of unqiue values.