def create_feature_vectorizer(input_features, output_feature_name, known_size_map={}):
    """
    Create a feature vectorizer from input features. This returns a 2-tuple
    ``(spec, num_dimension)`` for a feature vectorizer that puts everything into a
    single array with a length equal to the total size of all the input features.

    Parameters
    ----------
    input_features: [list of 2-tuples]
        Name(s) of the input features, given as a list of ``('name', datatype)``
        tuples.  The datatypes entry is one of the data types defined in the
        ``datatypes`` module.  Allowed ``datatypes`` are ``datatype.Int64``,
        ``datatype.Double``, ``datatypes.Dictionary``, and ``datatype.Array``.

        If the feature is a dictionary type, then the dictionary must have integer
        keys, and the number of dimensions to expand it into must be provided by
        ``known_size_map``.

        Feature indices in the final array are counted sequentially from the
        from 0 through the total number of features.


    output_feature_name: str
        The name of the output feature.  The type is an Array
        List of the output features of the network.

    known_size_map:
        A dictionary mapping the feature name to the expanded size in the final
        array.  This is most useful for specifying the size of sparse vectors
        given as dictionaries of index to value.

    """

    spec = proto.Model_pb2.Model()
    spec.specificationVersion = SPECIFICATION_VERSION

    input_features = process_or_validate_features(input_features)

    feature_vectorizer = spec.featureVectorizer

    num_output_dimensions = 0

    for n, ft in input_features:
        if n in known_size_map:
            dim = known_size_map[n]

            if ft.num_elements is not None:
                if dim != ft.num_elements:
                    raise ValueError(
                        "In feature {}, override size {} not compatible with inherent "
                        "value size {}.".format(n, dim, ft.num_elements)
                    )
        else:
            if ft.num_elements is None:
                raise ValueError(
                    "In feature {}, inherent size unknown so must be manually supplied.".format(
                        n
                    )
                )
            dim = ft.num_elements

        num_output_dimensions += dim

        new_feature = feature_vectorizer.inputList.add()
        new_feature.inputColumn = n
        new_feature.inputDimensions = dim

    if not isinstance(output_feature_name, str):
        if (
            is_valid_feature_list(output_feature_name)
            and len(output_feature_name) == 1
            and output_feature_name[0][1] == datatypes.Array(num_output_dimensions)
        ):

            output_feature_name = output_feature_name[0][0]

        else:
            raise TypeError(
                "Output feature must be specified as a feature name or correct output feature list."
            )

    output_features = [(output_feature_name, datatypes.Array(num_output_dimensions))]
    set_transform_interface_params(spec, input_features, output_features)

    return spec, num_output_dimensions

    Create a feature vectorizer from input features. This returns a 2-tuple
    ``(spec, num_dimension)`` for a feature vectorizer that puts everything into a
    single array with a length equal to the total size of all the input features.

    Parameters
    ----------
    input_features: [list of 2-tuples]
        Name(s) of the input features, given as a list of ``('name', datatype)``
        tuples.  The datatypes entry is one of the data types defined in the
        ``datatypes`` module.  Allowed ``datatypes`` are ``datatype.Int64``,
        ``datatype.Double``, ``datatypes.Dictionary``, and ``datatype.Array``.

        If the feature is a dictionary type, then the dictionary must have integer
        keys, and the number of dimensions to expand it into must be provided by
        ``known_size_map``.

        Feature indices in the final array are counted sequentially from the
        from 0 through the total number of features.


    output_feature_name: str
        The name of the output feature.  The type is an Array
        List of the output features of the network.

    known_size_map:
        A dictionary mapping the feature name to the expanded size in the final
        array.  This is most useful for specifying the size of sparse vectors
        given as dictionaries of index to value.

    
coremltools/models/feature_vectorizer.py