The lxml iso schematron implementation uses the RNG file unconditionally to check if the (standalone or XSD/RelaxNG-extracted) Schematron schema itself is valid: https://github.com/lxml/lxml/blob/397c0d94511f219bf7fa00ff71d2630e52fff5d2/src/lxml/isoschematron/__init__.py#L65-L67
The lxml iso schematron implementation uses the RNG file unconditionally to check if the (standalone or XSD/RelaxNG- extracted) Schematron schema itself is valid: https:/ /github. com/lxml/ lxml/blob/ 397c0d94511f219 bf7fa00ff71d263 0e52fff5d2/ src/lxml/ isoschematron/ __init_ _.py#L65- L67