Combine KML files with Python

You can use the fastkml python lib for this. It can be found at https://fastkml.readthedocs.io/en/latest/ with all instructions to the usage.

Here is a code example for using this library to generate a kml structure.

from pathlib import Path

from fastkml import kml
from shapely.geometry import shape


def read_kml(kml_text):
    k = kml.KML()

    # Reading the kml from a text string
    k.from_string(kml_text)

    k_doc = list(k.features())[0]

    # Getting the first document child that must be a folder or a placemark
    # but you can iterate over it to get multiple folders or placemarks
    doc_child = list(k_doc.features())[0]

    lst_geoms = list()
    if isinstance(doc_child, kml.Folder):
        for feature in doc_child.features():
            lst_geoms.append(feature.geometry)
    elif isinstance(doc_child, kml.Placemark):
        lst_geoms.append(doc_child.geometry)

    # Returning the geometries found list  
    return lst_geoms


def generate_kml():
    # Final file IO
    k = kml.KML()
    ns = '{http://www.opengis.net/kml/2.2}'

    # Creating document structure
    document = kml.Document(
        ns, "SOME_IDENTIFICATION",
        "SOME_NAME", "SOME_DESCRIPTION"
    )
    k.append(document)

    # Creating folder structure
    # Note that a kml can have multiple folders
    folder = kml.Folder(
        ns, "SOME_IDENTIFICATION",
        "SOME_NAME", "SOME_DESCRIPTION"
    )
    document.append(folder)

    # Creating placemark structure
    # Note that a kml can have multiple placemarks
    placemark = kml.Placemark(
        ns, "SOME_IDENTIFICATION",
        "SOME_NAME", "SOME_DESCRIPTION"
    )
    # Passing the geometry to the placemark
    # It can be a shapely geom (Polygon, MultiPolygon, Polyline, Point, etc)
    placemark.geometry = shape("SOME_GEOJSON_GEOMETRY_HERE")

    # Adding the placemark to the previous created folder
    folder.append(placemark)

    final_kml_text = k.to_string(prettyprint=True)

    output = Path(__file__).parent / "test.kml"
    output.write_text(final_kml_text.encode("UTF-8"))


if __name__ == '__main__':
    # U can either generate a kml from a existing data
    generate_kml()
    # or read another kml to get the features and properties
    # to generate your own
    geometries = read_kml("SOME_KML_STRINGIFIED_TEXT")

I experienced this problem just now and came up on this answer. I noticed the error message had pyKML with capitalized letters. It will work if you use all lowercase letters.