Normalizer and NaN values.

[w32zhong]

For StandardScaler, looks like it supports NaN values, see class Normalizer:

null_index = np.isnan(X)

However, during preprocess, _fill_na() will fill na_value for non-string.
So

• for dtype=str, the X values will be string
• for dtype=float/int, the X values will be na_value

In the first case, np.isnan will throw an error because X elements are of string type.
In the second case, there is no point to normalize numbers if we have a na_value there.

Is this behavior expected or not?

[zhujiem]

The bug has been fixed by removing null_index = np.isnan(X)
nan values are not allowed for numeric data type. users must set na_value explicitly.