为什么我在上面定义了变量时会收到“未定义的变量”错误

Question

Entire file

import os
import re

import pandas as pd 
import numpy as np

from tqdm import tqdm
import matplotlib.pyplot as plt 

import librosa

class ImportData():
    filename = "fma_metadata/tracks.csv"
    tracks = pd.read_csv(filename, header=2, low_memory=False)
    tracks_array = tracks.values
    track_id = tracks_array[: , 0]
    track_genre = tracks_array[:, 40]
    track_id = track_id.reshape(track_id.shape[0], 1)
    track_genre = track_genre.reshape(track_genre.shape[0], 1)

    # fma dataset 
    song_folder = "fma_small"

    sub_dirs = [o for o in os.listdir("fma_small")
        if os.path.isdir(os.path.join("fma_small",o))]

    count = 0

    print("Converting .mp3s into MEL spectograms")

    for d in sub_dirs:
        label_dir = os.path.join(song_folder, d)
        file_names = [os.path.join(label_dir, f) for f in os.listdir(label_dir) if f.endswith(".mp3")]

ImportData()

Here is my code where I am experiencing the error.

# fma dataset 
song_folder = "fma_small"

sub_dirs = [o for o in os.listdir("fma_small")
    if os.path.isdir(os.path.join("fma_small",o))]

count = 0
for d in sub_dirs:
    label_dir = os.path.join(song_folder, d)
    file_names = [os.path.join(label_dir, f) for f in os.listdir(label_dir) if f.endswith(".mp3")]

After I run my code, on line 33 it outputs 'name label_dir is not defined' even though I have it defined on the previous line.

Why is this error occurring?

Exception has occurred: NameError
name 'label_dir' is not defined
  File "D:\FinalProject\import_data.py", line 44, in <listcomp>
    file_names = [os.path.join(label_dir, f) for f in os.listdir(label_dir) if f.endswith(".mp3")]
  File "D:\FinalProject\import_data.py", line 44, in ImportData
    file_names = [os.path.join(label_dir, f) for f in os.listdir(label_dir) if f.endswith(".mp3")]
  File "D:\FinalProject\import_data.py", line 12, in <module>
    class ImportData():

Answer 1

You have executable stuff at the top level of a class definition. That it not necessarily wrong but, normally, you'd only expect to see variable assignments and method def:s there. Are you sure you didn't mean to say def ImportData() instead of class ImportData() ?

(Note to language laywers: Yes, I know that def:s and assignments are also executable...)