SimpleImputer strange behaviour

[Keyeoh]

Hi,

I am currently experiencing some weird errors while trying to port a pipeline from sklearn to dask. With respect to the SimpleImputer preprocessing class, I have noticed that the most_frequent strategy is currently implemented in such a way that the result is not what I think it should be. It reads:

def _fit_frame(self, X):
    if self.strategy == "mean":
        avg = X.mean(axis=0).values
    elif self.strategy == "median":
        avg = X.quantile().values
    elif self.strategy == "constant":
        avg = np.full(len(X.columns), self.fill_value)
    else:
        avg = [X[col].value_counts().nlargest(1).values for col in X.columns]
        avg = np.concatenate(*dask.compute(avg))

    self.statistics_ = pd.Series(dask.compute(avg)[0], index=X.columns)

My problem is with the following line:

avg = [X[col].value_counts().nlargest(1).values for col in X.columns]

I have been debugging this part and the previous line returns the counts for the most frequent item, not the item itself, which can introduce some weird errors when dealing with categorical data.

Does anybody have an explanation for this? Or did I miss something? Any hint will be much appreciated.

Regards.
Gus.

[TomAugspurger]

I think you’re correct, sorry about that. We should get the .index to get the item, not the count.