Understanding why this python code works randomly

Question

I'm coding a little script that gets metadata from a sound file and creates a string with the desired values. I know I'm doing something wrong but I ain't sure why, but it's probably the way I am iterating the if's. When I run the code :

import os, mutagen

XPATH= "/home/xavier/Code/autotube/tree/def"

DPATH="/home/xavier/Code/autotube/tree/down"


def get_meta():
    for dirpath, directories,files in os.walk(XPATH):
        for sound_file in files :
            if sound_file.endswith('.flac'):
                from mutagen.flac import FLAC
                metadata = mutagen.flac.Open(os.path.join(dirpath,sound_file))
                for (key, value) in metadata.items():
                    #print (key,value)
                    if key.startswith('date'):
                        date = value
                        print(date[0])

                    if key.startswith('artist'):
                        artist = value
                        #print(artist[0])
                    if key.startswith('album'):
                        album = value
                        #print(album[0])
                    if key.startswith('title'):
                        title = value
                        #print(title[0])
                        build_name(artist,album,title)  # UnboundLocalError gets raised here


def build_name(artist,album,title):
    print(artist[0],album[0],title[0])

I get the desired result or an error, randomly :

RESULT :

1967 Ravi Shankar & Yehudi Menuhin West Meets East Raga: Puriya Kalyan

ERROR :

Traceback (most recent call last):
  File "<stdin>", line 39, in <module>
  File "<stdin>", line 31, in get_meta
    build_name(artist,album,title)
UnboundLocalError: local variable 'album' referenced before assignment

Answer 1

If "title" comes before "album" in the meta data then album will never be initialised. "album" may not exist at all.

As you don't blank out the value of album for each track, if a track has previously had "album" defined then the next track which doesn't define "album" will use the previous track's value.

Give it a blank value for each track (if that's reasonable to you).

Looking at build_name the values are lists of strings, so the default should be ['']:

for sound_file in files:
    artist = album = title = ['']

However, you will still not get values before calling build_name if the metadata is out of order.

You need to move build_name(artist, album, title) out of the loop:

for (key, value) in metadata.items():
    ...  # searching metadata
build_name(artist, album, title)