Error Message: TypeError: unhashable type: 'set'

[twinpiques]

Quick and dirty solution with less Python knowledge:

hashable_data = tuple(set(ITERABLE))

Mutable objects don't have a hash, because they can mutate.
Immutable objects doesn't change, so they have have a hash.

There is also a built-in type, called frozenset and yes, it does what it sounds like.
This is an immutable set, which has an hash.

You can make the test:

# will fail
{set(): 42}

# is ok
{frozenset(): 42}

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Try this:

df_sar['sar_details_sent_norm_trigrams_unique'] = df_sar['sar_details_sent_norm_trigrams_'].apply(lambda x: frozenset([trigram for sent in x for trigram in sent]))

And you can remove the square brackets, then it's a generator expression, which is consumed by frozenset (saves memory).
Otherwise first a list from the set is created in memory, then it's applied to the dataframe.

df_sar['sar_details_sent_norm_trigrams_unique'] = df_sar['sar_details_sent_norm_trigrams_'].apply(lambda x: frozenset(trigram for sent in x for trigram in sent))

This is great - thank you.