DataType of InputField is double although in the PMMLPipeline it is string

I am exporting a PMMLPipeline with a categorical string feature day_of_week as a PMML file. When I open the file in Java and list the InputFields I see that the data type of day_of_week field is double:

InputField{name=day_of_week, fieldName=day_of_week, displayName=null, dataType=double, opType=categorical}

Hence when I evaluate an input I get the error:

org.jpmml.evaluator.InvalidResultException: Field "day_of_week" cannot accept user input value "tuesday"

On the Python side the pipeline works with a string column:

data = pd.DataFrame(data=[{"age": 10, "day_of_week": "tuesday"}])
y = trained_model.predict(X=data)

Miminal example for creating the PMML file:

import pandas as pd
from sklearn.compose import ColumnTransformer
from sklearn.ensemble import RandomForestRegressor
from sklearn.model_selection import train_test_split
from sklearn.pipeline import Pipeline
from sklearn.preprocessing import StandardScaler, OneHotEncoder
from sklearn2pmml import sklearn2pmml
from sklearn2pmml.pipeline import PMMLPipeline

if __name__ == '__main__':

    data_dict = {
        'age': [1, 2, 3],
        'day_of_week': ['monday', 'tuesday', 'wednesday'],
        'y': [5, 6, 7]
    }

    data = pd.DataFrame(data_dict, columns=data_dict)

    numeric_features = ['age']
    numeric_transformer = Pipeline(steps=[
        ('scaler', StandardScaler())])

    categorical_features = ['day_of_week']
    categorical_transformer = Pipeline(steps=[
        ('onehot', OneHotEncoder(handle_unknown='ignore', categories='auto'))])

    preprocessor = ColumnTransformer(
        transformers=[
            ('numerical', numeric_transformer, numeric_features),
            ('categorical', categorical_transformer, categorical_features)])

    pipeline = PMMLPipeline(
        steps=[
            ('preprocessor', preprocessor),
            ('classifier', RandomForestRegressor(n_estimators=60))])

    X = data.drop(labels=['y'], axis=1)
    y = data['y']

    X_train, X_test, y_train, y_test = train_test_split(X, y, train_size=0.8, random_state=30)

    trained_model = pipeline.fit(X=X_train, y=y_train)
    sklearn2pmml(pipeline=pipeline, pmml='RandomForestRegressor2.pmml', with_repr=True)

EDIT: sklearn2pmml creates a PMML file with A DataDictionary with DataField "day_of_week" that has dataType="double". I think it should be "String". Do I have to set the dataType somewhere to correct this?

<DataDictionary>
    <DataField name="day_of_week" optype="categorical" dataType="double">

Solution

You can assist SkLearn2PMML by providing "feature type hints" using sklearn2pmml.decoration.CategoricalDomain and sklearn2pmml.decoration.ContinuousDomain decorators (see here for more details).

In the current case, you should prepend a CategoricalDomain step to the pipeline that deals with categorical features:

from sklearn2pmml.decoration import CategoricalDomain

categorical_transformer = Pipeline(steps=[
    ('domain', CategoricalDomain(dtype = str))
    ('onehot', OneHotEncoder(handle_unknown='ignore', categories='auto'))
])