[scikit-learn] top N accuracy classification metric

[Johnson, Jeremiah]

Hi all,

It's common to use a top-n accuracy metric for multi-class classification problems, where for each observation the prediction is the set of probabilities for each of the classes, and a prediction is top-N accurate if the correct class is among the N highest predicted probability classes. I've written a simple implementation, but I don't think it quite fits the sklearn api. Specifically, _check_targets objects to the the continuous-multioutput format of the predictions for a classification task. Is there any interest in including a metric like this? I'd be happy to submit a pull request.

Jeremiah

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