def reverse_letters(sentence: str, length: int = 0) -> str:
 """
 Reverse all words that are longer than the given length of characters in a sentence.
 If unspecified, length is taken as 0

 >>> reverse_letters("Hey wollef sroirraw", 3)
 'Hey fellow warriors'
 >>> reverse_letters("nohtyP is nohtyP", 2)
 'Python is Python'
 >>> reverse_letters("1 12 123 1234 54321 654321", 0)
 '1 21 321 4321 12345 123456'
 >>> reverse_letters("racecar")
 'racecar'
 """
 return " ".join(
 "".join(word[::-1]) if len(word) > length else word for word in sentence.split()
 )


if __name__ == "__main__":
 import doctest

 doctest.testmod()
 print(reverse_letters("Hey wollef sroirraw"))