如何在循环中为每个类实例输出__str__

Question

I have a playlist of songs, and I would like to create an instance of the class SongData for each song in the playlist. I have code that isn't included that extracts the song name, artist, and song directory from the playlist. For the purposes of this question I have included these as 3 lists, songNames, artistNames, dirNames.

class SongData(object):
    def __init__(self, title="", artist="", directory=""):
        self.title, self.artist, self.directory = title, artist, directory
    def __str__(self):
        desc_str = "Title: %s \nArtist: %s \nDirectory: %s \n" %(self.title,
                                            self.artist, self.directory)
        print desc_str

songNames = ['song1', 'song2', 'song3']
artistNames = ['artist1', 'artist2', 'artist3']
dirNames = ['dir1', 'dir2', 'dir3']

songs = {name: SongData(title=name) for name in songNames}
artists = {band: SongData(artist=band) for band in artistNames}
directorys = {direc: SongData(directory=direc) for direc in dirNames}

I would like to be able to print out the desc_str for each song so it appears like this:

Title: song1 
Artist: artist1 
Directory: dir1

But so far I have only managed to call one data category at once, so it prints Title: song1 but leaves the Artist: and Directory: sections blank. I have been using:

print songs['song1']
print artists['artist1']

How can I get it to print them out all at once? I think the problem is that I am only referencing the data in each dictionary, not calling the instance of the class relevant to that song.

When I do it simply and manually, as follows, it works, but I cannot figure out how to put this into a loop so it will work for the lists songNames , artistNames , dirNames .

p = SongData('song1', 'artist1', 'dir1')
p.__str__()

Answer 1

Your SongData class needs to return from the __str__ function:

class SongData(object):
    def __init__(self, title="", artist="", directory=""):
        self.title, self.artist, self.directory = title, artist, directory
    def __str__(self):
        desc_str = "Title: %s \nArtist: %s \nDirectory: %s \n" %(self.title,
                                            self.artist, self.directory)
        return desc_str # <------ RETURN HERE

Then, you can build up instances of SongData as such:

songNames = ['song1', 'song2', 'song3']
artistNames = ['artist1', 'artist2', 'artist3']
dirNames = ['dir1', 'dir2', 'dir3']

songdata = [SongData(*params) for params in zip(songNames, artistNames, dirNames)]

Note that the above is a list-comp that given the above 3 lists is effectively equivalent to:

songdata = [
    SongData('song1', 'artist1', 'dir1'),
    SongData('song2', 'artist2', 'dir2'),
    SongData('song3', 'artist3', 'dir3')
]

Then loop over those, and use print to get the result of your __str__ :

for song in songdata:
    print song

Gives you:

Title: song1 
Artist: artist1 
Directory: dir1 

Title: song2 
Artist: artist2 
Directory: dir2 

Title: song3 
Artist: artist3 
Directory: dir3 

Answer 2

Or it is also possible to print a for loop within the __str__ function and then return empty string.

def __str__(self):
    for item in self.UserClasses:
        print item
    return ""

This works like a charm for me. Although might not be the best practice.