def buildDictionary():
    movieFile = open("movies.txt","r")

    #our dictionary
    movieDict={}

    #read line by line
    for line in movieFile:

        #break up the line
        lineList=line.split(",")

        #get actor
        actor = lineList[0]

        #get movies
        movieList = lineList[1:]

        #go through each movie, adding to dictionary
        for movie in movieList:
            movie=movie.strip()

            #put movies (key) in dictionary

            #movie not in dictionary
            if movie not in movieDict:
                movieDict[movie]=set()
          
                movieDict[movie].add(actor)

            #else movie is dictionary
            else:
                movieDict[movie].add(actor)
    movieFile.close()

    return(movieDict)

def compareTwoMovies(movie1, movie2):

    movieDict = buildDictionary()

    #print error if movie not in dictionary
    if movie1 not in movieDict or movie2 not in movieDict:
        print("Movie(s) not found.")
        return None

    #set operations
    set1 = movieDict[movie1]
    set2 = movieDict[movie2]

    #union
    print("The union is", set1 | set2)

    #intersection
    print("The intersection is", set1 & set2)

    #sym difference
    print("The sym difference is", set1 ^ set2)

def coActors(actor):

    movieDict = buildDictionary()
    outputSet = set()

    #go though each move, looking for actor
    for key in movieDict:

        #if value contains actor, add all actors to output set
        if actor in movieDict[key]:
            outputSet = outputSet | movieDict[key]

    #you can't be your own co-actor!
    outputSet.discard(actor) #discard won't throw error!

    #if the output is empty, we didn't find anyone
    if len(outputSet) == 0:
        print("No coactors found.")
    else:
        print(outputSet)