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Replace if letters == vowels[0:9]:
to if letters in vowels:
will solve your problem.
Simple explanation: ==
will check whether the left element is identical to the right element, in your case, to the left of ==
is a single letter, while to the right is "aeiouAEIO" (yeah, there's a capital U missing as well) and they can't be identical in any case.
Full programme:
horton = "A person's a person, no matter how small."
vowels = "aeiouAEIOU"
for letters in horton:
if letters in vowels:
print(letters)
# A
# e
# o
# a
# e
# o
# o
# a
# e
# o
# a