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Computing the $F_1$ score of a classifier for a specific class is a common task.

Desired solution

• New method called f1_score in Classifier
• Required parameter positive_class to specify which value should be treated as positive (all other classes are treated as negative)
• Call sklearn.metrics.f1_score

Formula:

$$ \frac{2 \mathit{TP}}{2 \mathit{TP} + \mathit{FP} + \mathit{FN}} $$

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