seb.py

from flask import Flask, send_file
from io import BytesIO

app = Flask(__name__)

@app.route('/get_placements')
def get_placements():
    placements = '<Placemark>bla</Placemark>'  # replace with your own placement creation logic
    f = BytesIO()  # memory buffer, acting as a file
    # f-strings are new in py3.6+
    content = f'''<?xml version="1.0" encoding="utf-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2">
{placements}
</kml>'''
    # the content string must be of type `bytes` in this context (not `unicode`, which is the default with py3+)
    f.write(content.encode('utf8'))
    f.seek(0)
    # not sure if `as_attachment` is needed in your context
    return send_file(f, attachment_filename="placement.kml", as_attachment=True)

if __name__ == '__main__':
    app.run(host='127.0.0.1', port=8080, debug=True)

# To test:
# python3 seb.py
#
# curl http://127.0.0.1:8080/get_placements
# <?xml version="1.0" encoding="utf-8"?>
# <kml xmlns="http://www.opengis.net/kml/2.2">
# <Placemark>bla</Placemark>
# </kml>%