…sthough'

Currently, if remainder='passthrough', then get_feature_names() will raise a NotImplementedError, but this pull request adds in that functionality. Now if the transformer is fit on a DataFrame, then the passthrough columns will appear in gte_feature_names() as the respective column names in the DataFrame, and if it is not a DataFrame then the column indices will be used instead.

While making the changes for ‘passthrough’, it seems sensible to make an additional change to ColumnTransformer.get_feature_names(), to make it always returns something. This implementation adds feature names name__x0, …, name__xN for transformers without a get_feature_names method.

It seems harsh to raise an error if there are some transformers which do have feature names, and even if none of them have a get_feature_names then it is still helpful to know which features came from which transformer.

…sthough'

Currently, if remainder='passthrough', then get_feature_names() will raise a NotImplementedError, but this pull request adds in that functionality. Now if the transformer is fit on a DataFrame, then the passthrough columns will appear in gte_feature_names() as the respective column names in the DataFrame, and if it is not a DataFrame then the column indices will be used instead, where the feature names will be 'xi' for the ith index.

…h' the same way

as remainder='passthrough'.

The behaviour of get_feature_names for passthrough is now the following:
- If fitted on a dataframe, then the columns passed with the trans='passthrough'
will be treated as positional if int (in which case the feature name will be
the column name at that position) or they will be used for the actual feature name.
Any columns in remainder='passthrough' will be appended to the end of feature_names,
as the column names from the fitted dataframe
- If fitted on an array, then the column names for both when trans='passthrough' or
when remainder='passthrough' will be 'xi' for each index value i. In terms of ordering,
the remainder columns will again come after the rest of the feature names.

Removed as this was waiting on PR #14495, which did not go ahead.

Fixed issue caused by missing import introduced when doing a merge in the browser. _check_key_type has been replaced with _determine_key_type.

Co-Authored-By: Thomas J Fan <thomasjpfan@gmail.com>

- Seperated the pandas part of the test into its own function to avoid the whole test being skipped when pandas is not installed.
- Removed the unused _output_dims attribute.

…rjball/scikit-learn into column_transformer_passthrough

Added support for mask and slices for both dataframes and arrays, as well as tests for each case.