Python: iterate over a nested dictionary

Question

given the following dictionary :

playlists={'user':[
               {'playlist1':{
                    'tracks': [        
                    {'name': 'Karma Police','artist': 'Radiohead', 'count': "1.0"},
                    {'name': 'Bitter Sweet Symphony','artist': 'The Verve','count': "2.0"}                    
                     ]
                    }
               },
               {'playlist2':{
                    'tracks': [        
                    {'name': 'We Will Rock You','artist': 'Queen', 'count': "3.0"},
                    {'name': 'Roxanne','artist': 'Police','count': "5.0"}                    
                     ]
                  }
                },
              ]
            }

though not very elegantly, I can fetch keys and values in such a structure:

keys :

userX = [user[0] for user in playlists.iteritems()][0] //userX
playlist1 = list(playlists['userX'][0].keys())[0] //playlist1

and nested values :

artist = playlists['user'][0]["playlist1"]['tracks'][0]['artist'] //Radiohead
count = playlists['user'][1]["playlist2"]['tracks'][0]['count'] //3.0

but I'm a bit confused as to how access all playlists names for playlists and all artists for tracks in playlists using any iteration tool.

how can I do it?

Answer 1

You can use following code to access the playlist names and artist names:

# Define generator to return dicts under each user
lists = (x for user in playlists.itervalues() for x in user)
print [name for play_list in lists for name in play_list.iterkeys()]

lists = (x for user in playlists.itervalues() for x in user)
track_lists = (content['tracks'] for p in lists for content in p.itervalues())
print [track['artist'] for track_list in track_lists for track in track_list]

Output:

['playlist1', 'playlist2']
['Radiohead', 'The Verve', 'Queen', 'Police']

Note that above works only with Python 2, with Python 3 values & keys should be used instead of itervalues & iterkeys .

Answer 2

If you are looking to extract a flat list of all playlist names you can use a list comprehension. The mix of dictionaries and lists adds a little complexity.

all_playlists = [ playlist_title 
                  for user in playlists 
                  for user_playlists in playlists[user]
                  for playlist_title in user_playlists ]
print all_playlists

all_artists = [ track['artist']
                for user in playlists
                for user_playlists in playlists[user]
                for playlist_title in user_playlists 
                for track in user_playlists[playlist_title]['tracks'] ]
print all_artists

gives

['playlist1', 'playlist2']
['Radiohead', 'The Verve', 'Queen', 'Police']

Note that this will give combined lists of playlists/artists across all users in the playlists dict