The Longest Palindromic

"""
Write a function that finds the longest palindromic substring of a given string. Try to be as efficient as possible!
If you find more than one substring you should return the one which is closer to the beginning.
"""

from functools import reduce

def longest_palindromic(text):
    result = []
    while text:
        _string = ''
        for letter in text:
            _string += letter
            result.append(_string)
        text = text[1:]
    result = [x for x in result if x == reduce(lambda a,b: b+a, x)]
    result = [x for x in result if len(x) == max(map(len, result))][0]
    return result

if __name__ == '__main__':
    assert longest_palindromic("artrartrt") == "rtrartr", "The Longest"
    assert longest_palindromic("abacada") == "aba", "The First"
    assert longest_palindromic("aaaa") == "aaaa", "The A"