pandas get unique values from column of lists

Question

How do I get the unique values of a column of lists in pandas or numpy such that second column from

would result in 'action', 'crime', 'drama'.

The closest (but non-functional) solutions I could come up with were:

 genres = data['Genre'].unique()

But this predictably results in a TypeError saying how lists aren't hashable.

TypeError: unhashable type: 'list'

Set seemed to be a good idea but

genres = data.apply(set(), columns=['Genre'], axis=1)

but also results in a TypeError: set() takes no keyword arguments

Answer 1

You can use explode:

data = pd.DataFrame([
    {
        "title": "The Godfather: Part II",
        "genres": ["crime", "drama"],
        "director": "Fracis Ford Coppola"
    },
    {
        "title": "The Dark Knight",
        "genres": ["action", "crime", "drama"],
        "director": "Christopher Nolan"
    }
])
# Changed from data.explode("genres")["genres"].unique() as suggested by rafaelc
data["genres"].explode().unique() 

Results in:

array(['crime', 'drama', 'action'], dtype=object)

Answer 2

If you only want to find the unique values, I'd recommend using itertools.chain.from_iterable to concatenate all those lists

import itertools

>>> np.unique([*itertools.chain.from_iterable(df.Genre)])
array(['action', 'crime', 'drama'], dtype='<U6')

Or even faster

>>> set(itertools.chain.from_iterable(df.Genre))
{'action', 'crime', 'drama'}

---

Timings

df = pd.DataFrame({'Genre':[['crime','drama'],['action','crime','drama']]})
df = pd.concat([df]*10000)

%timeit set(itertools.chain.from_iterable(df.Genre))
100 loops, best of 3: 2.55 ms per loo
    
%timeit set([x for y in df['Genre'] for x in y])
100 loops, best of 3: 4.09 ms per loop

%timeit np.unique([*itertools.chain.from_iterable(df.Genre)])
100 loops, best of 3: 12.8 ms per loop

%timeit np.unique(df['Genre'].sum())
1 loop, best of 3: 1.65 s per loop

%timeit set(df['Genre'].sum())
1 loop, best of 3: 1.66 s per loop