docs = ["doc1.txt", "doc2.txt", "doc3.txt"]
line = []

def word_search(docs, keyword):
    kword = keyword
    for i in range(len(docs)):
        file = open(docs[i])
        line.append(file.readline().rstrip('\n').split(" "))
        if word_finder(line[0], kword) == True:
            return print(docs[i], kword)
        line.clear()

def word_finder(line, kword):
    kword = kword.upper()
    for word in line:
        word = word.upper()
        if word == kword:
            return True

word_search(docs, "casino")