Mastering Technical Interviews: The Unique Characters Problem
Valeria Rozenbaum

How about this one:

from collections import Counter
def is_unique(string):
if not string:
return True # Empty string is unique, i guess?
return Counter(string).most_common(1)[0][1] == 1
# tests
assert is_unique(‘abc’)
assert not is_unique(‘abcc’)
assert is_unique(‘’)
assert is_unique(None)
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