First non-repeating character

DESCRIPTION:

Write a function named first_non_repeating_letter that takes a string input, and returns the first character that is not repeated anywhere in the string.

For example, if given the input 'stress', the function should return 't', since the letter t only occurs once in the string, and occurs first in the string.

As an added challenge, upper- and lowercase letters are considered the same character, but the function should return the correct case for the initial letter. For example, the input 'sTreSS' should return 'T'.

If a string contains all repeating characters, it should return an empty string ("") or None -- see sample tests.

 

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def first_non_repeating_letter(string):
    stringL=string.lower()
    place=0
    for i in stringL:
        if stringL.count(i)==1:
            return string[place]
        place+=1
    return ''