Regarding multi label classification

Question

I am performing multi label classification in python using sklearn. Here is the classification report

          precision    recall  f1-score   support

       0       0.77      0.67      0.71      7536
       1       0.76      0.77      0.76      6811
       2       0.84      0.84      0.84      5948
       3       0.78      0.75      0.77      4006
       4       0.96      0.94      0.95      3956
       5       0.70      0.60      0.65      3282
       6       0.85      0.70      0.77      3199
       7       0.74      0.68      0.71      3023
       8       0.64      0.57      0.60      2729
       9       0.92      0.85      0.88      1970
      10       0.75      0.56      0.64      1952
      11       0.98      0.93      0.95      1952
      12       0.88      0.81      0.84      1683
      13       0.79      0.75      0.77      1592
      14       0.75      0.64      0.69      1581
      15       0.75      0.68      0.71      1549
      16       0.84      0.69      0.76      1429
      17       0.70      0.63      0.66      1293
      18       0.63      0.51      0.56      1226
      19       0.71      0.50      0.59       993
      20       0.81      0.54      0.65       941
      21       0.61      0.35      0.45       815
      22       0.77      0.57      0.66       747
      23       0.83      0.57      0.68       752
      24       0.79      0.15      0.25       661
      25       0.73      0.63      0.68       526
      26       0.54      0.31      0.39       459
      27       0.66      0.44      0.53       450
      28       0.70      0.62      0.66       398
      29       0.78      0.09      0.16       229
      30       0.75      0.57      0.65       141
      31       0.75      0.22      0.34       108
      32       0.60      0.11      0.19       106

micro avg 0.79 0.70 0.74 64043
macro avg 0.76 0.58 0.64 64043 weighted avg 0.79 0.70 0.73 64043
samples avg 0.82 0.76 0.76 64043

/usr/local/lib/python3.6/dist-packages/sklearn/metrics/classification.py:1437: UndefinedMetricWarning: Precision and F-score are ill-defined and being set to 0.0 in samples with no predicted labels. 'precision', 'predicted', average, warn_for)

I don't understand the above warning . If the precision and F-score was ill-defined then the precision should be 0 for some class. But for all the values, precision and f1-score are values greater than 0. Why is this warning taking place? Am I missing something here?

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Answer 1

I don't know the framework but it seems that's what happens:

Since this is multi-label classification, it's possible that an instance is assigned no label at all by the model. Apparently at least one of the samples contains only instances which are predicted with no label. This means that there are no instances predicted positive for any label in this sample. This causes an NaN value for the precision of this particular sample, which in turn makes the average precision over all samples undefined (hence the error).

I assume that this is not related to the fact that all the classes have a positive precision and recall, since these values would be calculated over the whole dataset, not sample by sample.